The real reason Verizon and AT&T don’t want their customers to easily be able to unlock their boot loaders is data usage. People download apps like Wifi Free and go to town on the network. Less of a problem with people with capped data plans, but there’s still too many unlimited users who could bog down the network for everyone.
I have heard that AT&T detects unauthorized (aka unpaid) tethering, but its a lot easier for them to just never let you do it in the first place.