I don’t believe that DocumentDB actually runs any MongoDB code. On the other extreme, there aren’t any apparent Amazon-only features in their services that are simply hosted versions like they do with Redis (via Elasticache). If that was the issue these companies could have just gone with the AGPL. The problem isn’t that they’re maintaining internal forks with changes they won’t contribute back, it’s that they’re either (a) running unmodified versions or (b) cleanrooming the whole shebang.

I don’t see how to make the first impossible without effectively using a proprietary license, and banning the second would hurt open source much more than it would help it