I personally liked working at Amazon, and think most of the horror stories are from bad teams (there are also good teams!) but you pointed out one of the few downsides that applies across the board.

Amazon is not very friendly towards open-source. When I was there, I believe their policy was that you needed to ask for permission to contribute to any open-source project. The policy was broad (covering things like code snippets on StackOverflow or /r/programming on Reddit) and strict (I heard they would not approve anything related to game development). There definitely were folks who contributed to open-source projects -- I think those people either didn't ask for permission or had friendly managers.