T-Mobile previously offered tethering at no additional charge because the didn't have an official policy. The also didn't throttle you down until you hit ~10GB/month. But now, it's stated in their TOS with the throttle point set at 5GB/month for the same $30/month and they charge you the extra $15/month for tethering.
Why you ask? Well, a while back some people did a lot of whining and complaining. One such person by the name of Trent Alvarez, decided to file a class action lawsuit against T-Mobile over throttling--particularly on the issue as to why unlimited was not truly unlimited as it were. There were even those who supported Mr. Alvarez in his fight for the little guy because no big company is gonna push us little consumer around, right? Right. Well, the lawyers and Mr. Alvarez may get what they want (i.e. $$$), those that supported his cause get to be right, T-Mobile gets to charge more for the same service even if it has to state the limits more clearly now, but the rest of us are getting the shaft.