It is not an ad hominem attack to point out that the source of the information has previously shown to be biased and use monetary incentives to garner outcomes that fit their viewpoints.
It's not, OTOH, ad hominem to point out the same fact, for instance, as a caution to those who might uncritically accept the conclusions offered by the source.
Writing papers that usefully critique the IPCC reports is important and there's nothing wrong with compensating such work. The thing AEI did "wrong" was to cast a wider net in an attempt to find people better capable of doing that. AEI was doing something praiseworthy - trying to find critiques from outside normal places a conservative think tank might look, which might highlight overlooked points of view. If that's wrong, what's the alternative? Only use papers from people you have on salary? Only use papers from people who are so solidly in your own ideological camp they won't run to the Guardian? That's nuts! Shouldn't we WANT any scientists who have reservations about the IPCC conclusions to have an incentive to write papers exploring the question?