Yeah, and that probably didn't stem much of the flow because the rest were published by journalists. If anything some of the insiders' accounts were just as scathing as the explicitly anti-Trump books, which is saying something.
A non-trivial number of people do across the political spectrum. Political memoirs for instance are usually ghostwritten (outside of the first Obama book and maybe the Pete book given he was a nobody) and are only purchased in order to show support for the candidate. It's a team spirit thing.
Plus, I'm sure some have their own niches (satire, exposés like "Frankly we won this election" that have new reporting, etc) and many have their own historical value. I saw an ant-Tom DeLay book in the wild a few years ago well after his fall from grace and considered buying it just as a keepsake of the era.
I want to quickly emphasize that they are data driven enough to understand that lots of people buy those books because about 51-54% of the country doesn’t and never did like him.
Google's yearly raises are generally the same as Amazon I've learned, for what it's worth. The difference is the bonus and refreshers, and the fact that there are more evaluation periods with more achievement buckets.