This is incorrect. When you are a 'normal' XGB of high speed data plan your mobile device usage and tethered usage come out of the same bucket, you get 1-5GB (depending on plan) of high-speed data usage, after you hit your limit you are unable to use the Mobile Hotspot feature of your smartphone until your billing cycle rolls over.

Unlimited smartphone users are in a different situation, they have "unlimited" un-throttled mobile data usage, and 7GB of Mobile Hotspot usage.

To accurately measure Mobile Hotspot data usage, your phone connects to a separate data access point when Mobile Hotspot is turned on - but there are people who are using third-party tethering applications that simply use the 'standard' access point, allowing users on "unlimited" plans to skirt around the 7GB Mobile Hotspot limit.

T-Mobile EXPLICITLY advertises the "unlimited" plan only applies to usage ON YOUR PHONE, not on any tethered devices. They have every right to curb people using their service in this fashion.