I don't think that they can disable tethering on your phone, I'm pretty sure that they can't. Even if you enable it I don't think that they have a way to know. I'm thinking that maybe if you do a lot of traffic they might scrutinise you but if you keep it low (little browsing and small downloads) they can't figure out what you're doing as the phone acts as a router and all ongoing traffic is passed through it..
Sent from my HTC Desire S using xda app-developers app
Device: HTC Desire S [SAGA]
History: Bosch COM509, Ericsson T20e, Sony Ericsson T310, Sony Ericsson T610, Nokia 6600, Nokia 6630, Nokia 7710, Nokia N72, Nokia N95, Nokia N97mini, HTC Desire S, HTC One S