However, it would definitely piss off the affiliate program...who should just boot Google from their program.
Now that I think about it more, that's the real problem with this. The businesses are most likely getting sales anyway, and pinterest can't argue that they're doing anything special to generate the sale. Since pinterest can't censor links to these merchants, they'll probably end up getting booted from their programs.
You can't be serious. The sales would never have been generated in the first place if pinterest never existed. Your logic would seem to preclude any affiliate program in existence. If I have a blog reviewing my favorite books, and being a book lover I want to make it easy for people to enjoy books I recommend so I go ahead and place links to Amazon. Should Amazon say "well you we're going to link to us regardless, so you aren't eligible."
Currently, Pinterest doesn't have a foot to stand on if the affiliate programs simply kick them out. They can do things to bias certain merchants, but that's a riskier play.
That's true, the sale would never had been generated if Pinterest never existed. But the key point here is that the sale would still have been generated even if Pinterest was dropped from the affiliate program. The user generated link would still exist.
As a blogger you could say well I'm not going to link to any Amazon products now. Pinterest doesn't have that luxury. They could remove any links that they can't monetize but users would revolt since they are the ones adding the content and links.
Would you trust them not to?
Soon they're approaching companies and selling placement, charging listing fees, etc.
It's a business and their entitled to do this. As a user I'm entitled to ignore their offerings on the grounds that the way they ultimately profit is at my (the user's) expense. Eventually they're just internet noise with a slick interface.