While I hate the economic stifling of tech innovation that is the App Store culture, I would like to point out that Jailbroken iphone were used by the Chinese government to target protesters in Hong Kong quite recently.
My point is that stating that side-loading is a back-door that can be abused when your phone is in someone else's possession ends up not being a very good argument when that actually ends up being the harder way to accomplish getting software onto the phone, considering side-loading is both password protected (if your phone is) and still doesn't expose functionality beyond what the OS allows (unlike jail-breaking).
