The rule above is not from the App Store guidelines, but from the SDK license. I.e. even if you use alternative app store without restrictions (Cydia), but build a browser with Apple iOS SDK - it's still illegal, and that's why major browsers like Firefox wouldn't do it.

I'd say it's a very ugly practice, to use development tools license to ban competition. It's like saying buying this chisel you are bound to only carve logos of the chisel making company.