Mildly Infuriating

Net neutrality isn't going to do a thing about this kind of stuff. In a best case scenario, you'll end up with overall data usage limitations - no more 'unlimited mobile data'.

ISPs meter data usage because it's pretty much the only way they can impose some form of limitation on a finite capacity to provide such data to you and other customers - other than data rate limits (read: slower speeds). They can't guarantee data rates in almost any setup, because ultimately, while 'data usage' is a bit of an artificial construct and 'data' is not in any way finite, the pipes that deliver the data certainly are of finite capacity. Mobile data capacity - and in fact, any wireless medium - is a shared medium, the more people try to use it simultaneously, the less pleasant it's going to be for each individual user. Ask Starlink users in many US areas how overselling limited capacity impacts the individual user.

Mobile data usage also has different usage patterns than if you're hotspotting your PC. You're not going to download massive games or other bandwidth hogs to your mobile. You probably won't be running a torrent client either. So they can give you unlimited mobile data because you're simply not going to put as much of a strain on the infrastructure with pure on-device usage than you will with hotspotting.

This isn't a defense of what AT&T is doing. But net neutrality isn't going to force them to suddenly be all ethical. It's not going to make them provision infrastructure that doesn't fall over at the first signs of higher-than-usual load. And it certainly can't change the physical realities of wireless data communication. In an ideal world ISPs wouldn't be so greedy and/or beholden to greedy shareholders to be cutting corners, and instead provide sufficient infrastructure that can handle high demand.

And to those who are talking about their workarounds: you may not like it but you've signed a contract. That contract stipulates acceptable use, and if you're found to be breaching the contract terms, the other party is within their rights to terminate the contract. Again, in an ideal world these contract terms would be more balanced towards the needs of the customer, but in the meantime your best recourse against unfavourable contract terms is to take your business elsewhere. And if you can't do that, everything else is at your own risk.