Don't forget that Tesla has to have a service agreement with someone (AT&T here) for their 3G/LTE connectivity, and that someone probaby doesn't allow tethering for the rate that Tesla pays.
They aren't going to give you tethering for free, they are going to give you an option for say $400 a year to allow for a hot spot from the car. and they will give a portion of that $$ to AT&T for tethering.