Why should Amazon care? Nobody is going to stop shopping there. People just want their cheap shit, and don't care who they do business with. If most people could save $0.50 buying their toilet paper from known child rapists, they would.
There are practical reasons for having the federal government involved. Primarily, certain states (TX, AL), would be teaching kids about Jeebus and other Magical Sky Zombies if left to their own devices. The Constitution is nice in theory, but there are limits to its practicality.