You beat me to it! How do they get away with that?
in the case of unlimited broadband, if you have a, for example, talk talk package that is 'unlimited' for £20 per month and are like me, using it to watch iplayer, e-mails, forums etc then you are unlikely to have a problem with the limit that is in place that is termed 'fair use'
however, if you are a business doing tech support where you regularly use the internet to remotely access machines and downloading large amounts of data you will go over the limit, but then again, you're running a business, a domestic package is not intended for your needs, you need a business package.
the charges bundled in with the business package reflect more accurately your usage - you need more so should pay for more.
the limit is there to stop businesses using domestic packages and the provider loosing money through it.
the other reason is to try and curb downloading of films / music etc via file sharing programs or torrents both of which are not strictly legal.
the trick is to look at the fair use policy for the provider you are looking at going with, BT have told me off for using too much internet when talk talk have never done so.