From the state's perspective, probably along the same lines as why long guns are allowed with permit in many countries where handguns are banned.
You've got a supercomputer and a library and a set of video production equipment in your pocket, among other things. The capabilities of such a device are fundamentally different from something that's tethered to a desk or that's conspicuous when out-and-about. The idea of it being open and untrackable is exciting for some and terrifying for others.