That's all great. AT&T can throttle me down to 56k under their "fair use" policy if they want, just as long as they do not restrict the way I actually access data and the amount I use. This would be consistent with the plan I signed up for.
What I take issue with is the "You can't tether, because we've never allowed you to tether (but we'll allow it if you sign up for a plan that is exponentially more expensive)". It's just not the case that I wasn't allowed to tether under my original plan. I was, in fact, able to tether before, and now I've come across a situation where I might want to do it again. In the interim, the company has changed hands, but my data plan has remained the same.
The company now called AT&T (and their standards and definition of "unlimited") changed. My plan hasn't. I don't think it's unreasonable for them to honor the contractual obligation they inherited, which they (as well as I) have maintained for more than a decade now.