Paid links are black hat SEO. If you sell links you should be putting rel="nofollow" on them.

Also Google does do a pretty rigorous job of combatting paid ad spam. They have many ad text and landing page policies designed to weed out sneaky affiliate deals and the like. They also reward advertisers that provide more popular user experiences through their quality score mechanism which is a direct (and very significant) factor in cost per click.

It could be AppGratis was just as rigorous in their evaluation of offers - I also don't think its a very valid analogy nor do I disagree with you generally save for those 2 points.