Many modern emulators need a BIOS-like ROM/flash image to even boot up; the situation is much less clear there, since these usually contain software and can accordingly be copyrighted.
Some emulators solve this by providing their own (re)implementation, but that’s not always easy.
And even when it’s “just” iOS: That is copyrighted too! So it’s the same problem – either there needs to be an iOS ABI-compatible OS, or it’s probably not possible to emulate any application legally, unfortunately.
