Yes they do, and yes they have.
I'm on the T-Mobile $30 prepaid plan, and have activated tethering on the plan three different times. Each time, the charge was prorated from the $15 monthly rate and then deactivated at the end of the billing cycle. Unless they changed this recently, tethering is only included with T-Mobile's Simple Choice plans, not their prepaid plans.
UPDATE:
Okay, I just checked. The Mobile Hotspot is actually active on my phone! Must be a recent change, because I last activated tethering on my phone two months ago. And at the end of the billing cycle, it was deactivated. Unlike with past tethering deactivations, the option on my settings never went away.