Think it's because the Torch doesn't support the 3G band used by T-Mobile. So although he's paying for a 3G plan, he can only use Edge.
I did the same to use my Storm2 on Rogers. Sucked not having 3G, but was well worth it for the phone :)
My 2cents on the T-Mobile/Torch debate - it will never happen! The form factor was conceived by AT&T, and the device was built as a collaboration with RIM over 16 months. There is no way AT&T would let that go after only couple months of exclusivity.