Hotspot was one of the features originally promised for the Model S. It made sense that the feature was withdrawn when the Model S only had 3G service.
With the upgrade to LTE, there is enough bandwidth that a hotspot would be useful. Assuming the hardware is capable of supporting a hotspot (changing the WiFi from client to access point mode), the restriction would likely be in the agreement Tesla has with AT&T for providing LTE service to the Model S - no hotspots - and Tesla is currently not charging owners for the 3G or LTE service.
If there was a charge for upgrading to hotspot service, it's likely Tesla would find some owners interested in paying that fee - to gain back the hotspot functionality that was originally promised on the Model S.
Though Tesla also hasn't delivered on other promises that were made for the Model S - such as the App Store or providing annual map updates...