Why were slave owners even offered compensation via the 1837 Act if they'd lost the power in parliament to stop abolition in 1833?

Lord Wynford, though speaking in opposition, illustrated the reasons in the House of Lords, on Tuesday, June 25th 1833, why a payment was being considered and what arguments such a payment should be based on. I've quoted from that below, but the arguments he makes are:

• the crime of slavery has been a crime committed by everyone so everyone should pay for it;
• "the money voted is necessary for the support of the honour or interest of the nation";
  • this point can be misconstrued as implying that slavery was honourable; instead the point is that it would have been dishonourable to retroactively legislate on items:

    Consider, for example, if (for environmental reasons), it was suddenly made illegal to own a petrol or diesel powered vehicle, and they would all be taken away by the Government. If compensation is not given, then it would be considered akin to an ex post facto law (punishing people for an act which was legal at the time, but since made illegal) - that's the "dishonourable" bit (and, in fact, is currently prohibited under Article 7 of the ECHR). i.e. "We can't punish people for not predicting we would later move the goalposts"
    _{—Chronocidal, in comments.}

• to compensate for the owners potential loss they "may sustain from not having any means of cultivating their estates";
• to account for reduced production of sugar in the colonies which cause losses for the plantation owners;
• previous attempts at emancipation without mentions of compensation had failed to pass in local assemblies.

Lord Wynford on June 25th 1833:

We say you are proceeding with so much haste, and so incautiously, that instead of doing any good, you will do much mischief ; instead of putting an end to a system of cruelty and injustice, you will increase it ;—that whilst you emancipate slaves who live under English laws, you will increase the numbers of those persons in countries in which they will not have the protection of those laws ;—that whilst the endurance of a state of slavery has been the crime of the whole British nation, and, therefore, the loss that may arise from putting an end to this detestable state should be equally borne by us all, we are proceeding to legislate without any proof that the compensation which is to be made to the owners of slaves is anything like a fair equivalent for the sacrifice of property that they will now be required to make. ... It is first proposed that we should resolve that 20,000,000£ should be granted to the West Indian proprietors, for giving up their interest in their slaves. Compensation to slave-owners will remove my strongest objection to immediate emancipation. I heard, with great satisfaction, the Noble Lord who began this debate state, a few days ago, that he would not consent to emancipation unaccompanied with compensation to the slave-owners. I only did the Noble Earl justice when I said, upon that occasion, that I was sure the Noble Earl was too candid a man when he talked of compensation, to mean the compensation that has been mentioned in another place, which was,—taking a part of the services of the slave, and by money raised by the employment of the part of the services so taken, compensating for the whole of the services left for a time to the master. No, instead of what was originally proposed, namely, a loan of 15,000,000£ to be repaid by property fraudently taken from the labourer, the slave-owners are now to have an absolute gift of 20,000,000£. We ought, on the other hand, to take care that the compensation made them be a just compensation. I suspect, from a few words that fell from the Noble Earl, that 20,000,000£ will not be a just compensation. He said something about paying for the value of the negroes. If you only pay for the negroes, and make no compensation for the injury that the owners may sustain from not having any means of cultivating their estates, you will give them nothing like a just compensation. If you direct the stream from a mill, do you compensate the owner by paying him for the water, without giving anything for the mill, which the taking of the water has rendered useless ?

... It is not only the right but the duty of both branches of the Legislature not to vote for the imposition of any new burden on the people, until they are satisfied that the money voted is necessary for the support of the honour or interest of the nation. ...

In that answer of my Noble Friend to the Noble Duke, he seems to admit that so much sugar will not be produced in our colonies after the emancipation of the slaves as now. Indeed, if that is not expected, where is the ground for compensation? But I must tell my Noble and Learned Friend, that although the same quantity of sugar should be consumed in Great Britain, and the duties on importation be the same as they are at present, if sugar is not cultivated in our islands the revenue must be diminished. The lands in these colonies can be used for no other purpose but the cultivation of sugar. If sugar cannot be cultivated on them they will be useless to their owners, and the compensation proposed to be granted for the loss of their slaves is nothing like an equivalent. The proposing such a compensation is offering them an insult at the moment that you are accomplishing their ruin. They will have nothing to export to Great Britain, and they will be unable to pay for any imports from Great Britain. ...

As to the refusal of the colonial legislatures to emancipate slaves, was it ever proposed to them to make them a compensation for this sacrifice of the property of their constituents? No such proposition was ever made to them, and they have, therefore, done right in refusing to pass a law which would have the effect of making their constituents bear the whole loss that must attend the measure. If the 20,000,000£ be anything like a fair compensation for the loss they must sustain in one part of their property, and the danger they are in of having all their property rendered of no value, I am persuaded they will emancipate the slaves. It is unjust to them to propose these resolutions to Parliament before any offer of compensation has been made to the colonists.
_{—'The Debates in Parliament, Session 1833 - on the Resolutions and Bill for the Aboliton of Slavery in the British Colonies'}