LTE would increase bandwidth consumption because customers would be more likely to use high-bandwidth services (eg: satellite images in maps instead of vector graphics), and less likely to have the car connect to a LTE hotspot.
Personally, I have my iPhone programmed to turn on the Personal Hotspot every time the car's bluetooth connects to the phone, and have the car switch over to that hotspot as I drive. T-Mobile LTE is much much faster in my area than AT&T, and with my T-Mo unlimited data package, I figure I can save Tesla a few bucks while improving my service.
If it had LTE, I would probably have less of an incentive to tether. I'm sure there are other people who tether for similar reasons.