That's exactly what this is: Google is trying to prevent tech illiterate users from installing malware.
(Or at least, that's their take on this. You can choose to read between the lines, or not, as to whether they have other motivations also.)
Of course they have other motivations
But for 1 person wanting to run their own software there are hundreds of people with the potential to install malware/crapware/etc
Define malware.