This is pretty par for the course, though. Who else is incentivized to determine what is "good enough" wrt user experience/safety on Google's platform? Microsoft, Apple, Canonical, Samsung, Steam, Amazon, etc are all the arbiters of what's "good enough" (or safe enough) for their respective platforms (and marketplaces) because it's their brand at bat when something _isn't_ good enough (or safe enough).

"Android" (by Google) takes a reputation hit when someone downloads malicious APKs from anywhere, including Google's store. Google, therefore, wants to reduce the availability of bad apps in their ecosystem; maintaining an app store that enforces their standard of quality is one approach to accomplishing this goal, dissuading unknown apps they can't vouch for is another.

Obviously, there are both pros and cons to these approaches (from both Google and the end-user's perspectives), and they could definitely be improved, but they accomplish a concrete goal of "reduce malware on Android".