The text on the hi-res version actually looks worse in Foxit at 100% than the original download
Somehow I missed this when I first read your post. If Foxit were degrading display in an attempt to convince you to purchase their program the difference would consistently be between Foxit and Acrobat. There would be no reason to degrade only hi-res images to the point they display worse
than low-res images in the same program. I'd put my money on some sort of problem in Foxit's display algorithm rather than a nefarious plot.