Hmm....interestingly enough, this only seems to affect residential plans.
Of course most business plans do not mention that tethering is even against the rules, so AT&T may have little recourse with them.
As for is this would be something that would allow you to leave without paying the ETF, I would say yes, as it does increase your costs. Either by decreasing your productivity (many people that are on residential plans still use the phones for business purposes) or by requiring you to find some additional method to access faster data, which can cost money as well.
In addition, this is a material change to the contract, after all unlimited means without restrictions, this is a restriction.
Now if there offered a new plan that was called something like 3GB High Speed Data plan with no overage costs, then that would be truthful and I would have no problems with that, but if it is called unlimited, it should be unlimited, end of story.
As far as grandfathering the plan in, in many states (if not all), they are legally required to do so.