Thin margins are a sign of an efficient free market. Any long lasting large margin is a sign of a market failure.

Maybe Amazon is just really good at that free market thing.

I agree though that it's starting to be time they get a more viable competitor. I'm sure, however, that when they decide to actually try and make a profit instead of just grabbing market share, competitors will pop up. In the mean time, while they continue to operate at zero profits, basically as a charity to consumers, this is a win for efficient free markets and consumers (read everyone) in general.