I think that they have it setup on their side when the server validates the session to check to see if the account allows for tethering. I had tethered my blackberry, which included unlimited data, for years for free. Didn't need it for a few months and went to use it yesterday and the session would not allow for a connection to anything other than a t-mobile splash screen telling me to add the service for $15 per month.

I now have a tablet for $25 per month to replace that device. Sucks to be T-Mo.