I find Google's actions to be bizarre. Getting people to link to you is what SEO is all about. Basically every site concerned with SEO does it in one form or another -- I thought this was what most every blog affiliate program was about! So Google decides to "punish" RapGenius just because its program got a little bit of public notice, while 99.9% of other sites which do the same, continue doing the same thing, unpun…
Amazon and iTunes aren't paying you for the link they are paying you for the sale.
In the second and third cases it is no different if you're paying for a keyword link and if Google found out about it you would be penalized.