Exactly. Things would look a lot different if Amazon was an active contributor of improvements to MongoDB. Plenty of companies are actively involved in the open source projects they use. It often makes a lot of business sense too: it helps the project develop in the direction they need, and by sharing their changes in that direction, they ensure that there's a community that supports the features they need, and from which they can hire experienced developers if they need to.

It makes a lot of sense, especially for smaller companies. But Amazon in this case doesn't use MongoDB for something they need, they just want to make it available to their customers. They have no opinion on what their customers need from MongoDB, so they're happy to just tag along for free.