Phones now have the same level of processing power as portable computers. We shouldn't be obligated to only run applications that are siphoned thru a third party "trusted clearinghouse", be it the smartphone or the operating system vendor.
If I want more safety or I am not a Power User, I can flick a switch on the device to allow that binding behavior to happen. Heck, they are giving me the 'free service' of taking care of app security for me, I could even pay for that if I am serious about having my apps checked and stuff.
Phones are computers. You own the hardware, you should be able to install whatever you want. Always.
Is the argument about protecting the masses of non-tech people? Great, make it an opt-out. And still: There ought to be someone keeping tabs on that big brother (gov regulations? agencies) or else shady behavior can ensue as well.
I pay a lot of money for iPad Pros and Google Pixelbooks for precisely this functionality. When the user can run anything they want, the result is the unsafe landscape of malware we see on common desktop OSes.