What are common reasons for US immigration officers to deny entry in the country?

CBP tells us that in FY18 (fiscal year 2018):

At ports of entry, CBP officers encountered 279,009 inadmissible individuals. While the most common reason CBP officers determined individuals to be inadmissible pertained to their inability to satisfy documentary requirements, CBP officers found some individuals to be inadmissible based on previous immigration violations, criminal grounds, and for national security-related reasons.

(emphasis mine)

This is of course quite a broad "reason".

DHS have more to say:

In 2019, 55 percent of the inadmissibility determinations occurred at land ports, 18 percent at air ports, and 27 percent at sea ports; these proportions are comparable to 2018; however, not comparable to 2016 and 2017 due to a temporary policy change in how crew members detained aboard were categorized in those 2 years. The leading ports were Laredo (where OFO found 50,000 aliens inadmissible), San Diego (35,000), El Paso (26,000), and Houston (25,000).

Most aliens found inadmissible by OFO at POEs fall into one of three main categories:

First, most inadmissible aliens from the leading countries of nonimmigrant admissions — including Mexico, Canada, People’s Republic of China (China), and India — are denied for having missing, invalid, or expired documents, for having intentions prohibited by the visa (e.g., presenting a tourist visa but intending to seek employment), or for national security reasons. These denials of admission constitute a small fraction of persons who present themselves for inspection at a POE.

Second, certain inadmissible aliens present themselves at a POE despite knowing that they are inadmissible in order to seek some form of humanitarian relief or protection. Historically, a large share of these aliens was paroled into the United States for humanitarian reasons or as a matter of policy.

Citizens of Cuba were generally exempted from the provisions of section 235(b)(1) of the INA under the former “Wet Foot – Dry Foot” policy, and many Cubans requested asylum at a POE, including many inadmissible Cubans not in possession of valid travel documents. With the rescission of this rule on January 12, 2017, the number of Cubans found inadmissible fell from 20,000 in 2017 to 9,400 in 2018; however, the trend of Cubans found inadmissible reversed and increased to 22,000 in 2019.

An increasing number of nationals from the Northern Triangle also sought asylum at POEs and were found inadmissible in each year from 2013 to 2018, but all three countries had decreases in these determinations in 2019.

Inadmissibility determinations of Northern Triangle nationals totaled 19,000 in 2019, a 50 percent decrease from 2018 (Figure 4). Inadmissible nationals from Northern Triangle countries who claim a fear of persecution or torture or who indicate their intention to apply for asylum may be placed in removal proceedings and either detained or released into the United States depending on available resources and other factors.

The third main category of inadmissible alien consists of crew members of foreign vessels who may be required to remain aboard their ships. Cargo operations can require visits to multiple ports, or multiple docks within a single port, and can take longer than the 29 days permitted by a D-1 nonimmigrant crew member visa. In such cases, crew members initially granted shore leave may be re-coded as inadmissible once the shore leave expires, regardless of whether the crew members intended or attempted to disembark the vessel. Most inadmissible nationals from the Philippines and China are in this category.

(emphasis mine)

If you are arriving by air, then a lot of checks will have been performed before you see a CBP officer (actually, before you are even permitted to board):

• you will have applied for a visa or an ESTA
• the airline will have checked you have a valid passport
• the airline will have checked you have a valid visa or ESTA
• the airline will have sent your details to CBP via API, and CBP will have returned a board/do not board response

So in that case, the most prevalent reason is most certainly having intentions prohibited by the visa (e.g., presenting a tourist visa but intending to seek employment), as well as intending to immigrate and stay in the country rather than leave before the term of you visa.

Note that of course, your actual intentions are not really relevant, only the perception by CBP of your intentions.

Other possible reasons involve national security, as well as having lied on your visa or ESTA application about inadmissibility reasons such as past convictions, etc.

A visa is just something that you need to enter the US. The visa does not indicate any decision to let you enter. It just allows you to be present for a decision to be made!

The visa is needed so that someone can make the decision: it's a necessary but insufficient part of the process. The actual decision is made each and every time by at the CBP post where you enter the US. Typical reasons for denying entry include:

• attempt to contravene immigration laws, e.g. applying for a tourist stay and then unauthorized employment, or giving reasons to suspect that such may be attempted in the future,

• prior actions in contravention of immigration laws, e.g. overstaying the entry permit duration without extension or advising someone how to "cheat" on a visa application; In other words: consequences of helping others skirt/break the law are at least as bad as doing it yourself is,

• prior criminal charges and/or judgments, whether in the US territories or outside,

• determination that there are grounds to believe that the entry of the individual is contrary to the interests of the US (or sometimes also its allies),

etc.

Note that the initial authorized duration of stay is tangential to any dates on the visa. It has everything to do with the decision made at the port of entry. The CBP officer will write in your passport and/or on your entry card how long you're authorized to stay. This may be shorter than what you expect in certain cases. E.g. just because the visa allows a stays of a certain duration, that's the upper limit. The actual length of stay authorized is determined on a case-by-case basis each time one enters the country, and an authorization may be denied as well of course. Say, a B1/B2 visa has a maximum length of stay of 6 months. I've seen people get permission to stay for 2-4 weeks if it was determined that that's all they need for their scheduled activity within the country (say a conference/symposium). At the same time, lying about what you want to do is just as bad and is grounds for refusal of entry, so don't try to "pad things". If you don't know how long it may take, or if you have potential extensions to your plans, just say so. You may be asked to provide some documentation that you have the means to afford your stay, or are otherwise financially supported, in any case.

And the flipside: just because someone's visa is expired doesn't mean they are not authorized to stay in the US. An immigration status may be adjusted without any physical visa being added to the passport, but other documents will substantiate such claims, e.g. a decision to extend or to adjust an immigration status. Such decisions are of course not awarded willy-nilly - there are copious regulations around that.

A visa is not a right to enter a country. Citizenship is that.

When you enter a country, two things happen.

• An immigration officer speaks to you as you enter.
• The customs service looks at your stuff.

Those people have access to all your data in the computer, so they know all about your visa application. If the conversation or your stuff suggests an intent contrary to law or your visa conditions, that could lead to a refusal. For instance, on a UK border-patrol TV show, someone's luggage was found to be full of business clothing and 20 copies of the person's CV. That was prima-facie evidence of "seeking employment", not allowed on their visa or VWP, so back they went.

So it is possible to have a valid visa, and then "put your foot in your mouth" at the entry interview, or be seen bringing in something foolish. For instance, it's a lousy time to try to smuggle cigarettes - people still do that, and still get turned away. (Trust me: they have cigarettes in stores here.) What makes it smuggling is "not declaring it" - declaring it greatly improves consequences for honest ordinary mistakes.

Because America is a federal country, this can blind-side people since we have a cacophony of differing government laws. Many states are simpatico with marijuana use, but it's a Class 1 narcotic to the feds. So drug paraphernalia or the smell of pot will doom you, even if it's legal in the state you are landing. (This is especially a problem at the Canada border.)

Speaking of that, border guards can prohibit things that are legal in both places - Canada won't let in marijuana and paraphernalia, even though it's legal in Washington state, and legal federally in Canada. A laptop full of p**n is a problem here.

Note that they can also inspect your phone and your social media activity both for contrary-to-visa activities and simply being undesirable... and if you say "well, I won't give them my passcode!" they will simply refuse you entry. So data hygiene is a good plan... also, not being a jackass on social media they know about.

Any visa is never an automatic permission to enter the USA, and that’s clearly stated in many places during the application.

The core reasons to get rejected are that the officer has the impression that you are either

• going to violate the limits of the visa,
• lied in the application, or
• have a criminal intent.

The details depend on the visa—with a tourist visa, if it seems you are planning to work it is a problem—and it does seem that you are planning to work if you don’t seem to have enough money.

Most visas also have a limit for the duration of your visit, and if you don't have a return ticket, or only a vague idea where you will go, it looks fishy, etc.