Steam SOLD me a game. Amazon SOLD me an e-book. Shut down my account or refuse to do further business with me as you like, that's your right, but if you SOLD me something, you can't have it back. You need to either make it available to me or return my money. If Amazon wants to put the word "buy" on a button, then something needs to be sold. Otherwise the button needs to read "acquire revokable license" or something.
And, of course, this leads to the reason it'll never happen: first sale doctrine. If I own something, I need to be able to transfer it to someone else.