Google has no obligation to provide the OSS version, but they can't really afford to close Android either, it would be a major PR blow and Samsung would basically immediately ship their own fork, perhaps with the Amazon store and a couple extra apps to replace the Google maps, Chrome, etc.

Manufacturers are limited contractually. This is detailed in the EU press release linked to this thread. Alas, one still has to actually read it or alternatively have some knowledge about how Android is licensed.