My take is that Fuchsia is Google's attempt to unify their mobile ecosystem under one proprietary OS like Apple does with iOS (yes, yes I hear you, "But Darwin is OSS..." try making that into iOS though...): which is a good move for Google.
They can do that regardless of the license since they own the copyright.
As long as they do not accept external contributions or require a CLA it does not matter what license they offer, they can still make future versions proprietary.
Even if they accept contributions normally (e.g. under the same license), the above licences will allow them (or anyone else) to make a proprietary fork.
