Interesting. I was just on the phone with Aio and they said they don't allow tethering. If you have a tablet with a SIM card they have a 250MB $15 monthly plan ($10 for additional 1GB of data).
I guess in their TOS they have given themselves the ability to take it away at any time. All I can say for now in practice is that it works. At least on Windows Phone and most Androids, I understand iPhone tethering is problematic.