Imagine if Microsoft did this on PCs. a) prohibiting the installation of non-windows store software (sideloading) and b) insisting that all purchases done via apps give them a 30% cut. I think this is a ridiculous practice on the behalf of Apple.
By maintaining control over which apps can be sold to end users via the App Store, Apple is able to offer a layer of control over how these apps can access (or not) your personal data.
There's nothing ridiculous about wanting to maintain data privacy/security on mobile devices, and that being of a higher priority with mobile than with a PC.