I'm pretty sure it would be illegal for them to do that. for them to know how you are doing the tether, they would basically have to do a tap on your data and capture your stuff....without a warrant...i think that's a bit illegal.
No; without a warrant it'd be illegal to disclose that info to a 3rd party requesting it, but if you read your TOS they certainly do not give up their right to log your traffic or do deep packet inspection. This is pretty common amongst ISP's.