They need at least to make it optional. In my case, I don't even trust 100% on the apps on the appStore, so I would not even consider to install things from 3rd party sites. But if somebody wants to do it at they own risk, I think that is perfectly fine. Apple needs just to properly inform the user of the risks of enabling this option.
Or, maybe the real problem for Apple is not security and simply to avoid people getting the (paid) App Store apps for free from other sources.
Or, maybe the real problem for Apple is not security and simply to avoid people getting the (paid) App Store apps for free from other sources.