Not sure why branding links are allowed here. One day Google will hit all those sites for sure. I think this has to be changed. I understand that this is the way for authors to earn extra money, but better provide additional resource or anything for the 10-20$ you charge. Imho, if you have thounsand do follow, sitewide links pointing to your site from various unrelated websites, guess what? Google penalty is coming. One way or another, sooner or later.