Distributions like Debian, Fedora, Ubuntu, etc. require that all software in their core repos be FOSS. It's not about their legal obligations but about their own policies forbidding proprietary software in their core repos.
Since the new Mongo license violates rule 9 of the DFSG, it is considered proprietary software by Debian's definition, and that means Debian's FOSS policy will require Mongo to be moved out of the core repos and into the non-free repo. Fedora and Ubuntu have their own policies which will have the same effect.
Mongo already provides their own repos, and recommends using that for install and updates. I don't think they care or anyone seriously wanting to develop something on/around Mongo does either.
(Thanks for the explanation, "DFSG" passed over me the first time I read this. "Debian Free Software Guidelines".)