After massive pushback. Their original plan was basically full control. It still is, but they'll allow you to install something if you ask nicely first.
The other issue is the timing. They can claim this is for security all they want, but it was announced suspiciously close to the courts ruling that Google needed to open up their ecosystem to other app stores. This is a blatant attempt to keep control of the app ecosystem by forcing devs to go through Google regardless of where they intend to release.