It's often easier to prove this (or, even more often, tax evasion) than prove the alleged criminal activity. Either way, bad guy goes to jail, so the government is happy.
While counterfeit stuff are sold by sellers in the marketplace that Amazon provides, that is not equivalent to Amazon being the one selling the stuff.
I'm sorry, but as far as I'm concerned, Amazon is selling the item.
Substance over form.
2. There is so much counterfeit stuff on Amazon that they should know about it, and should be held responsible since they do nothing about it. "Should have known" is sufficient for guilt in a court of law, when the thing is blatantly obvious.