Not really, we seem to be able to authenticate multiple iTunes accounts per device. So for apps we all use, I buy them and everybody uses my login for that one thing. Kind of a little cumbersome but it works ok.
That isn't how Amazon works - they will drive Roku out of business by pricing their box at zero margin, and Roku will lose money at that price point because Amazon will have greater clout with manufacturers and eventually higher volume.But they are no threat to AppleTV at all.
They could compete - I think that they made a mistake not supporting Amazon Prime, Roku would be a lot less popular if they had done it, and Amazon probably wouldn't have bothered with their own box.I don't see the point of supporting Netflix and not supporting Amazon Prime - block both if you are worried about cannibalizing iTunes, or allow both, but don't just allow one of them.
Yawn - they are not competing with Apple, but with Roku.
People buy the AppleTV to steam iTunes and/or Netflix, they buy the Roku to stream Netflix and/or Amazon prime. The overlap of a new Amazon box will mainly be with the Roku.