I'm pretty sure you could jailbreak the device and install the ipa from somewhere. And it's not a weakness of the iPhone. It a reason you personally wouldn't buy one but for the vast vase majority of people this will never ever matter.
I meant in the sense that by enabling that feature there are also security issues introduced to the platform and for most people those costs outweigh the benefit gained by adding the feature. So if you look at it as a missing feature you could consider it a weakness. If you look at the effects of introducing it to the platform, overall, leaving it out is not a weakness - it's a smart decision.
Sideloading doesn't really introduce security issues. Security against malware on iOS is provided by sandboxing, not by Apple's approval process, which is far too cursory to detect any serious malware.
Rubbish. It's about as much of a risk as letting a user do anything. Google even have a service that scans installed apks for known bad ones if you want to share everything you do with them. Apple could do something similar if they decided user choice wasn't evil and bad after all.
