They should either have one cheaper contract with said limits and one more expensive contract which is really unlimited instead of saying it is unlimited while in reality it isn't.
A lot do and I'd say that is part of the problem.
For example BT I think (their site seems to be under construction at the moment...) have 3 tariffs. One has a limit or 2GB or something, another more expensive one 20GB and the third called "Total" has an unlimited amount (which is limited to 100GB which you are only told about when you get to it... that's not mentioning the decidedly dodgy counting they do).
Now to most that would indicate that the unlimited connection is unlimited, because the rest are limited...