This is the most clearly anti-competitive practice.
Google supports AOSP and has done so for years, making it available freely. Why shouldn't they be able to dictate their own terms? If phone makers don't like it, they can make their own OS (which they have - and they all suck).
> Google has prevented manufacturers wishing to pre-install Google apps from selling even a single smart mobile device running on alternative versions of Android that were not approved by Google (so-called "Android forks").
Hence the fine.
You cannot have something open and control it at the same time.