This occurred to me the other day. I've always laughed at the idea that Apple blocks sideloading for security purposes, but if the first line of defense is and always has been security through obscurity + manual App Store review (>= 2.0) on iOS, it's very possible that sideloading could cause problems. iOS didn't even have an App Store in release 1.0, meanwhile the Android security model has taken into account sideloaded apps since the very beginning [1]:
> Android is designed to be open. [...] Securing an open platform requires a strong security architecture and rigorous security programs. Android was designed with multilayered security that's flexible enough to support an open platform while still protecting all users of the platform.
[1] https://source.android.com/docs/security/overview
Edit: Language revised to clarify that I'm poking fun of the idea and not the one who believes it.