Why shouldn't unlimited mean unlimited? For six months my only Internet connection was through my Blackberry. T-Mobile is very good about not restricting what you can do with your phone, and not complaining when you do things like this.
Seriously...this is like debating with the people who think it's okay to bring Tupperware to an all-you-can-eat buffet.
It's more like management kicking you out after they've felt you've had "all-you-can-eat".
Whether it is ethical to enforce a no-tether policy is a separate debate.