First, Although from the retaliation stand point, people shouldn't be using it but any company would prefer to keep one cloud service for their whole stack unless there are specific features which are available on Atlas but not on DocumentDB.
Second, it's surprising that no cloud service were supporting MongoDB specific VMs (I dont know the legal reasons behind it). Only MongoDB Atlas was the prominent player in the market --- which sounds like a monopoly to me and Amazon loves to break the monopolies regardless of the domain.
Third, I've used MongoDB Atlas ~1.5years ago and it was decent for running prod applications then. They weren't providing much options for guaging performance bottlenecks etc. but I don't know the current state though.
Finally, MongoDB as a Company doesn't engage themselves with Startup ecosystem but instead focusing more on Enterprise level and I've noticed that the adoption of MongoDB is incredibly high in blooming startups. All the startup accelerators and incubators provide huge amount free credits sponsored by AWS which will really reduce these startups' running cost if they adopt DocumentDB.
Although Amazon doesn't give back anything to OSS community(which they should) but introducing MongoDB support in their Cloud Service indirectly benefits Startups and Enterprises to an extent.
I'd be curious to know thoughts from other people too.