[tb]

It's called oversubscription and it's a very common practise. Statistically, not all of the ISP's customers are going to use all of their quota every month, so why should the ISP provide enough bandwidth to allow them to? That just means they're spending far more on infrastructure than is really necessary to service their customers' actual requirements.

Also, why should the quota be tied to the speed? When I want to download a movie, I want to download it as quickly as possible, but that doesn't mean I want to max out my bandwidth continually.