A third party browser should be able to do the same things as Apple's browser. Same for all apps: they should have access to the same features of the device as Apple's apps.

Apple's rationale in their letter about this was essentially that they would need to ensure that it works (create and review use of a framework for non-Safari-backed PWAs in iOS) and that (usage of PWAs) x (percent of userbase affected) was too small for them to bother.