There are some hosts that simply advertise things that they cannot offer. A lot of these hosts will have stipulations buried in their TOS pages that don't relate to bandwidth, but that they can easily snag anyone on. These are little backup plans they use as an excuse to deactivate an account, usually without refund, for whatever reason they want to. This way they are not known for kicking sites off for bandwidth reasons, but rather for a TOS violation. No Unsure by CgiScripts.
Hosts use false advertising because other hosts do it.
They figure that the majority of consumers do not understand how little bandwidth they really need, and will never know that it is not really unlimited. Unfortunately, there are those who do use it and learn this lesson the hard way. Unless you are offering software or mp3 downloads or are just extremely popular, you shouldn't have to worry too much about bandwidth.