What is the last point where one can throw away fruits if one has indicated "not bringing any fruit" on the US customs form when flying to the US?

Let’s be clear about the laws:

OP’s question tilts on whether the "bright line" is the US border, or the CBP immigration desk.

You can’t bring them into the United States

US law 7 USC 7712 gives the Secretary of Agriculture (a)broad authority to "prohibit or restrict the importation, entry," etc. "of any plant"… and (c) issue regulations to implement this, including (c)(1) permits. Congressional intent here is clear, but as is typical, they leave the details to the department. Let’s see what the Secretary has done with it.

7 CFR § 319.56-3 General requirements for all imported fruits and vegetables.

(b) Permit.

(1) All fruits and vegetables imported under this subpart, whether commercial or noncommercial consignments, must be imported under permit issued by APHIS.

Whoops. You don’t have one of those, so that’s the end of that. Regulations (CFR) are law, for all practical purposes.

In their consumer-facing dumbing-down of the CFR, USDA says

Almost all fresh fruits and vegetables (whole or cut) are prohibited from entering the United States because of the potential pest and disease risks to American agriculture. This includes fresh fruits or vegetables given to you on your airplane or cruise ship. Please plan to leave them behind.

That’s clear enough.

Catch the part about "given to you on your airplane"? One implementation practicality is that airlines serve food. They probably don’t run the permit process for it. So another reg allows this, as long as it stays on the plane. When you bring your own food and abandon it on the plane, you are shaded by that rule.

The ruling words are "United States" which is a nation with clearly established borders and a 12 mile nautical limit. It plainly does not mean the CBP desk; thinking otherwise is wishful thinking. The plane, sitting on the tarmac, is in the United States.

That is the rule. OP asks what the rule is.

Why do some immigration points (Atlanta) have bins in advance of the Customs point, even though that should be illegal? Two reasons. First, this is normal in lawmaking: Congress writes laws (stated in USC) giving the agencies broad leeway in implmentation (both the regulations written in CFR and the actual field execution). Likely they ran into a practical overwhelm of people needing to throw things away, so this was just the easiest way to do it. Second, CBP (much like the IRS) loves traps. So don’t put it past them to have a video camera aimed at the bin.

So if you want to comply with the law, the bright line is the airplane door. Any further and you’re depending on implementation practices of CBP, which are not reliable.

You didn’t ask about Preclearance in a foreign country, but that’s a different deal because it’s enabled by a different law (19 CFR 162.8). Entering preclearance areas, you shall "comply with all U.S. Customs laws and other civil and criminal laws of the United States relating to importation of merchandise" which brings to bear the above laws. Up until then you’re in the foreign country’s law, which is why there, yes, the line is the immigration desk because you are not in the U.S. And under Irish law right up to the desk.

With greater power comes greater responsibility

Global Entry isn’t just some government hoop. It’s a program whereby you identify yourself as a more sophisticated traveler who is volunteering to be more knowledgeable about the rules and follow them. Quid pro quo is that CBP gives you lighter oversight at the customs desk.

You eagerly collect your half of the bargain, which is less standing in line and less being treated like a common tourist.

But you need to pay your half, which wasn’t the $100. It is to follow the immigrations and customs rules to a higher standard than they expect from a casual traveler. You agreed to be the person they don’t have to worry about.

As seen elsewhere, agricultural items are top of the list of concern – so much so that they’re mentioned separately (even though they’repart of "customs"):

Reasons for Ineligibility [for Global Entry]

• Have been found in violation of any customs, immigration or agriculture regulations or laws in any country;
• Cannot satisfy CBP of your low-risk status.

These are things you’re expected to know. And respect.

A rotten idea in the first place

Wondering over where exactly in the immigration line to throw away your fruits entirely misses the point of the anti-fruit regulations: To prevent the spread of invasive species — plant diseases, spores, bugs and bug eggs which travel on fruit, etc. — which can do a whole lot of damage to crops. Some people disbelieve that invasive species are a real problem; ask Australia. Or a botanist.

What if you had bare fruit in a bag, and the insect eggs or fungal spores transferred onto the bag, which you then carry through customs? What if the fruit went into the bin before Customs, but the eggs hatched and the insects flew away before the trash got burned? Point is, if you think about the reason for the rule, these are unnecessary risks.

All trash on international-arriving aircraft, in the disembark area, the bathrooms, the aisles, and Customs is all treated as dangerous biohazard, and burned. But the burn process is not 100%! So the best thing is to not even bring prohibited materials on the plane.

That said, the CBP question is asking what you brought into the USA, not what you brought past the imaginary "admitted to USA" line at the immigration desk. At the risk of stating the obvious, when you are standing in the immigration queue at LAX, you are in the USA. The airplane on which you abandoned your fruit is also in the USA.

Of course, if you tick "yes", you’ll stand in a longer line. I gather that’s what you want to avoid. OK then, don’t bring fruit.

It will depend on the exact border post. In airports you’re likely to have a bin somewhere between the plane and passport control. When taking the bus you might not have a big enough bin until you’re past immigration.

However at the end of the day it doesn’t matter in the slightest. If you have some fruit with you, simply mark yes on the customs form and dutifully show it to the customs agent as you hand back the form. If they want you to dispose of it, they’ll tell you where the closest bin is. If they don’t, you can keep your fruit and enjoy it later. It is absolutely legal to bring any food products to the border as long as you openly declare everything.

Source: bringing food at least 10 times over the US border.

Anything, such as fruit, vegetables, flowers is taken off the plane

• they must be declared on the Customs Declaration form

So the last point to throw anything away (abandon) is the plane, when indicating “not bringing any fruit” on the US customs form.

Customs is the last place where forbidden items, such as fruit, vegetables, flowers etc., which may contain ‘hidden guests‘, should be reported.

You can also strike through the ‘No’ and check the ‘Yes’ on the form.

You are well advised to make it vocally clear, before the routine Customs check starts, that you have such items to avoid any misunderstandings.

It used to be, that such items could be given to the flight personal, since they know how such items should be properly disposed of. Many things in the plane itself must be disposed of in the same manner.

In the area between the plane and customs, available garbage bins should be avoided. One may assume that in this area care is taken to depose of it properly, but the likelihood is greater that something may slip through.

• they may not be used for items that must be presented to a Customs Officer

The same is true for the European Union, where in most cases the same items are also not allowed to be brought in.

• Mangoes and Bananas from Manila to Switzerland

Bringing Food into the U.S. | U.S. Customs and Border Protection

…
All agriculture items must be declared and are subject to inspection by a CBP Agriculture Specialist at ports of entry to ensure they are free of plant pests and foreign animal diseases.
…
Declared agriculture items, in non-commercial quantities, that are found to be prohibited or restricted by the CBP Agriculture Specialists can be abandoned at the port of entry should the traveler wish to continue into the U.S. However, undeclared prohibited agriculture items will be confiscated and can result in the issuance of a civil penalty to the traveler for failure to declare the prohibited item. All agricultural items that are abandoned or confiscated at ports of entry are destroyed in accordance with USDA approved destruction methods to prevent spread of pests and diseases.

Agricultural pests and diseases are a threat to U.S. crop production and to the livestock industry. Some animal diseases can be highly contagious and could cause severe economic damage to livestock and result in losses in production, which could lead to increased costs for meat and dairy products. Plant pests and disease, as well as invasive plant material can cause crop loss and also damage lawns, ornamental plants, and trees. Plant pest infestations can result in increased costs to consumers due to pest eradication efforts as well as lower crop yields. High risk plant pest and animal disease outbreaks within the U.S. could also adversely affect the economy as a result of reduced trade of U.S. origin goods to countries around the world.

USDA/APHIS Fruits and Vegetables Import Requirements (FAVIR)

Passenger Entry

Can I take fruit on the plane when I travel?
Yes, but you must consume all of it on the plane. Any leftovers must remain on the plane and will be properly disposed of by the airline under APHIS guidelines.

When I am traveling, how can I use FAVIR to determine whether small amounts of fruits and/or vegetables can be brought into the U.S.?

A traveler must declare all fresh fruits and vegetables to a Customs and Border Protection (CBP) Officer upon arrival into the U.S. The CBP Officer must confirm the product name and origin. Non-admissible items will be seized, including those of questionable origin or use. Traveler’s returning from overseas likely cannot meet this standard, so should not carry fresh fruits and vegetables for personal use. Even if the article is admissible per FAVIR, CBP requires inspection for freedom from pests, diseases, and prohibited plant parts or contamination. Any commodity that requires treatment or other certification will not be allowed into the U.S. Failure to declare all fruit or any other food products could subject the traveler to a fine of $300 to $1,000 US dollars.

Conclution:
If anything is taken off the plane:

• it must be declared on the Customs Declaration form
• it must be presented to a Customs and Border Protection (CBP) Officer for inspection

Prohibited and Restricted Items

Fruits and Vegetables
…
One good example of problems imported fruits and vegetables can cause is the Mediterranean fruit fly outbreak during the 1980s. The outbreak cost the state of California and the federal government approximately $100 million to get rid of this pest. The cause of the outbreak was one traveler who brought home one contaminated piece of fruit. It is best not to bring fresh fruits or vegetables into the United States. However, if you plan to, contact either CBP or check the Permits section on the USDA-APHIS Web site for a general approved list on items that need a permit.

Due to the overreactions by some:

• bringing fruit to the US has been a cardinal sin at a time where
  • 90% of the US population had never heard of the word marijuana
• Shannon Airport, Ireland is not within the United States
  • but bringing undeclared fruit through the US-Customs there will nevertheless be considered an offence