This is essentially the same on MacOS now if you distribute, things built without signature at all only open on the machine they were built, you need to provide even a self signature to get it to open with a warning on another machine.
I feel like the thinking is that there must be an entity — somebody running an app store — who could be held legally responsible for any damage caused by malware distributed via their channels. Regular non-tech-savvy users cannot be trusted with such delicate software as apps running on their personal phones.
The only difference is Apple has the $$ and incentives to remove it as soon as it's brought to their attention (assuming it's actual malware that may cause large financial loss not just copyright infringement).
Alt-stores will be ridden with malware and nobody is going to be legally responsible for it. We can just hope the alt-stores that end up existing have incentives to keep them "clean".
If so, one would think that unless Apple gets to dictate terms strongly to the App stores, that this would only be a matter of time.