I think unethical is the word your looking for.
But some people make themselves feel better by sugarcoating it as a 'loophole.' Those same people will grab their torches and pitchfork and decry the evil corporation if they try to pull one extra nickel out of them or tweak the TOS.
Maybe. Maybe not. It's AT&T's own policy to grandfather data plans to anyone moving from the original iPhone to new(er) iPhones.
But who cares? It's hardly like these companies are looking out for us.