T-Mobile confirmed the memo to Engadget, and added an official statement with more details:
A very small number of our customers are misusing their Simple Choice Unlimited data service in violation of their rate plan and terms and conditions by bypassing the default tethering feature or engaging in peer-to-peer (P2P) file-sharing.
This type of usage can negatively impact our ability to offer affordable unlimited data. In order to protect all T-Mobile customers, we will be reaching out to these people to educate them on our terms and conditions of service, but if the misuse continues, they could have their data speeds reduced for the remainder of their billing cycle.
If you are on a T-Mobile unlimited data plan and download files via Bittorrent or use a tool like PDANet to get unlimited tethering to your laptop or other devices, be prepared to get "educated" and/or throttled by T-Mobile.
All of T-Mobile's current unlimited data plans are for on-device data only - tethering & hotspot for sharing with other devices is capped at a certain amount per month.
It is certainly understandable to see T-Mobile try and better manage the heaviest users on its network, but for those of us who depend on mobile plans as our primary connectivity these changes are frustrating, and make T-Mobile less appealing as a secondary data source for RV users.
Unlimited data options are getting rarer by the day.