And how could they actually see the difference - forwarded data to PC versus used in the phone?
The phone makes the distinction because the phone sets up the tethered access point. Unless the user installs apps to get around this, which is what they did.
As soon as the data traffic has reached the phone it's up to the phone owner to do whatever he/she want.
Not if the phone owner wants to abide by the contract they agreed to in their cellular plan. T-Mobile wants to have additional control on tethered data because it is easier to use more data on a computer than on a phone, typically.
Not saying it's right or wrong, but that's the reasoning for limiting tethered data, and a customer needs to go out of their way to get around the limits that they agreed to in the cell phone plan terms.