"and pinterest can't argue that they're doing anything special to generate the sale."

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."