The fact of the matter, is that Google usually publishes their source. They're just a bit slow at it. Most versions of android ship in binary form before source is released.
AOSP is fairly basic and somewhat limited on its own, unless you have the resources to replace all of the "gapps", including setting up your own app store. Google Play Services has been slowly taking over a lot of core (and not-so-core) phone functionality over the past year or more, and it's entirely closed source.
Google even stopped open-sourcing new versions of Google Authenticator, which you'd think would be a prime candidate for a full-blown open source project. (And hell, it's a crappy app; there are better GAuth-workalikes available.)
yet, they are just phasing all the important bits to privative code. Of course, this Android for work goes with Google Play, and guess what, it is privative, of course.
Google even stopped open-sourcing new versions of Google Authenticator, which you'd think would be a prime candidate for a full-blown open source project. (And hell, it's a crappy app; there are better GAuth-workalikes available.)