In addition to the ability to reconfigure your modem to only operate at certain speeds, service providers have other mechanisms at their disposal to control bandwidth usage on a per user basis.
For example, a few years ago I was a hardware engineer with a company and built a product that would sit "inline" between users such as yourself and the services providers' internal network and the Internet. Each and every packet traversing between users and the Internet could be observed, tracked and counted.
With such a box deployed, if a user's traffic patterns did not comply with the service providers policies, bandwidth could be limited for that user. And, this traffic monitoring wasn't just on a per user basis, but rather policies could also be created and enforced based on the type of traffic, i.e. peer to peer versus BitT**** versus http versus email and so on.
So, even though many of us might have multi-megabit Internet connections, say 3Mbit down or whatever, it's quite likely that the service provider may limit something like BitT**** traffic to something far less than that.
There are several companies building such products, the product I built being just one of them.
In addition to the ability to limit bandwidth on a per user, per protocol basis, these boxes can also do lots of other interesting things, like looking at any byte in any packet that they want to, for whatever information that is of interest. Google "deep packet inspection" if you want more information in that regard.