This mostly begins and ends with fear of liability. Amazon has lots of assets, tiny margins, and a lot of deep-pocketed competitors more than happy to fund legal battles by proxy, to say nothing of smaller trolls. (What other company is going head-to-head with Walmart, Microsoft, Google, Bertelsmann, and Apple in different market sectors at the same time?)
Restricting "official" contributions is really about limiting the legal attack surface.