Maybe they should be exact and say "Unlimited Internet within a certain bandwith quota" because, bandwith is expensive and if you pay 10 bucks for a plan with unlimited internet and someone else is paying... for the sake of argument lets say 50 bucks for same speed, but a set quota. If you go over or within a logical amount that would basically ruin that persons investment it would cost them money, and lose the company money. It's basically a way for them to get money, but at the same time limit how much bandwith you are using. Not to mention most of those "Unlimited Use" contracts always say something about it not being Unlimited.
(PS if that doesn't make any sense it's probably because I haven't slept. )