Phones already have app permissions: can access you contact, can access your location...

But no major phone OS provides a reliable "can access the internet" permission (without jailbreak/root). This would solve this issue much above the stack. I can install the dubious app. If the app can't access the internet at all (properly enforced by the OS) then by definition it can't leak anything.

I find it particularly disappointing from Apple. If they were truly committed to privacy as they claim, this would be a feature already.