I don't understand why tethering is part of the conversation. If a customer consumes 2TB of data a month, does it matter if they were tethering? They could be streaming video all day on their device. It shouldn't matter how the data is used, only how much of it is used. I'm certain it doesn't affect T-Mobile network on the type of data being transmitted.
I believe T-Mobile is using the word tethering to mask the fact that the plans are not really "unlimited". The plan is only unlimited with certain restrictions, which will could be changed as T-Mobile sees fit.