Because thumb preview TGPs generally have significant more traffic than a text-based TGP, they do manage to get surfers that are lucky enough to have a gallery on the first click.
I don't understand it either, there are better and easier ways to get sales rather than jerking surfers around. Treat others like you want to be treated, is my business model (I hardly ever used CJ-based sites, and leave all my sites and sponsor sites console-free).