Huh? Apple doesn't disable sideloading. If they did, it would be impossible for anybody to develop mobile iOS apps, or test apps for other developers. I have a few sideloaded apps on my iPhone right now.
I think you're fundamentally misunderstanding the malware problem with iOS/Android. It really isn't an issue with sideloading, as far too few people take the time to do that sort of thing. What it is is a problem with apps in the official stores that do things the user doesn't expect, such as record and report their personal information. The improved permission scheme in Android M should make this a bit harder, as most malware will have to have a reason the user understands for a specific permission.
I think the bigger reason that iOS has less of a problem here is that Apple has a much more stringent system for getting apps onto the app store than Google does.