The argument to force side loading is the argument that Apple are using market position in phone hardware sales to control, and profit from, the adjacent market of software that runs on phones.

Yes, you can run software on phones via a web browser. But Apple largely control the features of the web browsers that run on phones, too, and arguably limit those features so that browser apps are never quite as good as native apps.

This is "gatekeeping" and while it isn't illegal in all jurisdictions, it is anti-consumer, hence the consumer backlash.