Well, from a business standpoint, there are a lot of companies planning to piggyback off of Android to launch their own OS platforms. These competitors represent a real threat to the Android platform. When Google was all alone in the smartphone OS space (chief competitors being BBOS and iOS, which aren't licensed to 3rd party manufacturers) they didn't have to worry about competing for the attentions of manufacturers... companies got to choose between Android or making Symbian featurephones. So it was easy for Google to be generous with the source.

Now that Google has to worry about de-Google'd Android products, they need to keep more of their work for themselves.