They can force manufacturers to use unlocked bootloaders if they want the official Google version.
But hardly anyone is running the stock Android anyways. They've all expended on the code, and made it different. Almost no one actually uses the "official Google version" at all...
There's no reason whatsoever why Google can't make the same thing a requirement
But they can't because it's open source software. No one could make all of the Linux Distros use the same official kernel... it's not possible, because it's open source, and you can make it yourself.
and would prefer shipping a version of Android with no Google services
But this is already the case. Amazon and Barnes and Noble do not sell Android devices with Google services on them.
I don't get why people have such a difficult time understanding that Google can't wrangle these cats, because it doesn't have an Iron Fist on the source code...