T-mobile, unlike many providers, is actually allowing real and true unlimited data usage on phones. They've always had restrictions on tethering in their terms of service. You may or may not like it, but those are the terms you sign up with. If you don't read the full ToS, that's your fault, not theirs. If you want to violate those ToS, you're the one breaking the agreed-upon deal. Don't be surprised if they say you're not abiding by the agreement and act accordingly.
I'm a heavy tethering user. I'm also using a T-mobile unlimited 4G plan, and I intend to keep it. Historically I've used CynaogenMod, which automatically disables the tethering flag, though I'm not using it at the moment. I hope people using CM don't have problems because of this and get in trouble. I wonder if they'll only apply it if you go over the 7GB tethering cap?
The numbers they're quoting are really amazing for heavy users. Even when traveling and tethering I rarely break 5GB for ALL data for the whole month.