I don't know, why do we need a term to distinguish brown from dark orange? The term emerged organically because the built-in app store is the most common way to install apps on mobile phones (and the only way on iOS), but on Android you can also install apps from other sources without needing Google's permission so people came up with a catchy name for that.
It's convenient because now we can say "Google is killing sideloading" as a very succinct way to describe what's happening when we're arguing against it. "Blocking users from installing apps not approved by Google" works equally well but is a bit more wordy. I personally prefer the latter because I think it's a little more precise, but trying to imply people have to phrase things that way or they're part of some conspiracy does nothing but alienate your supporters and distract from the real issue.
It's convenient because now we can say "Google is killing sideloading" as a very succinct way to describe what's happening when we're arguing against it. "Blocking users from installing apps not approved by Google" works equally well but is a bit more wordy. I personally prefer the latter because I think it's a little more precise, but trying to imply people have to phrase things that way or they're part of some conspiracy does nothing but alienate your supporters and distract from the real issue.