1. they are not able to distinguish "purchased" or "not purchased" links
I'm not so sure about that..
All an Advertiser has to do is click on the link their advertising on...In the backroom manager area of that particular text-link company...and wallaa...now google analytics see's where that 'referring url' came from. And is now suspicious of that site paying for links to increase their PR.
Or vice-versa....a Webmaster clicks on an Advertiser's link VIA his backroom manager in that particular text-link site.