I do understand why they might be reluctant to offer tethering on an unlimited plan. But there is a technological solution: limit the amount of data through the tether. Make it $20/month for 2GB of tethered data. Or even just allow a user ("customer?") switch on a monthly basis from an unlimited plan to a limited plan with tether.
Don't want to dork around with jailbreaking. Isn't really all that important.