Windows is proprietary though. If Google starts dictating too much of what you can and cannot install on Android then they'll get dinged with another anti-trust suit.
They don't have to dictate it any more than Microsoft does (since they allow PC manufacturers to install all sorts of horrific nonsense). They just have to make the OS such that you can uninstall any application. If this is impossible to do with non-proprietary software, then that's a point for proprietary software.
The OEMs will just add the uninstallable flag functionality back because Android is open source.
The only way to prevent OEMs from doing that is to tie it to the Google Play Services agreement which will, again, open Google up to yet another antitrust suit.
If this is the case legally, then it seems in the world of phones, an OSS operating system puts users at the mercy of more organizations (OS developer and phone manufacturer) rather than giving them more control.
