I would assume it has to do with that fine print....they aren't really selling an "unlimited" plan, it's really 23GB of LTE speed and then something slower after that. VZW's plan were unlimited data, that later converted to LTE.
At least with TMo, I think it's literally something like 23GB of LTE, and then unlimited 2G or whatever.
With VZW, they WERE allowed to block tether on unlimited plans, because it was expressly excluded. But on their limited plans they could not do that.
It's very clear that none of the carriers are selling a replacement for home broadband, and ALL of them take steps, legally, to prevent that (unless your usage is low, i.e. little to no video).