Apple: why iPhone jailbreaking should not be allowed

In the US, the FCC. They're the ones who set the regulations for use of wireless technologies, so it's they who should determine whether modifications to a device are "broadcast legal" or not. And lucky for us, they already have.

So, really, any modification which does not change how the device transmits and receives radio waves in such a way that it violates FCC regulations for that class of device should be allowed.