A lot of people are confusing speed (bandwidth) with VOLUME. The speed part is what actually costs the companies more to get to your computer.
Once that first MB goes over the line though, the next 1,000,000 GB cost fractions of pennies. That's the volume part.
What they do is allow X amount of speed (bandwidth) from each central box (by subdivision or apt complex) - on the old "assumption" that not everyone connected to that central box will be online at the same time doing stuff. So they can keep "most" of the customers happy most of the time because the bandwidth isn't being maxed out. What has happened is that you get a few hogs, who use the speed to max out and slow down others on that same central box. Since more and more people are streaming, and not just from their computers, but their iFruits and iPods, even the home that uses their computer for just email and web surfing, their two kids are streaming stuff whenever they are home. Once that bandwidth of that nearest central box is maxed, everyone else slows down.
They are using this to try to "force" those few hogs to limit their usage (volume) or pay more, when they won't upgrade those central locations with more bandwidth. But yet they still keep offering more high speed connections which means you will hit your limit faster, and with most people, when you get that higher "speed" plan - you figure that now you can download and stream MORE - which would kinda make sense, and the companies are doing what they can to profit off of it - when they aren't upgrading their infrastructure to handle the additional users that connect up at the speeds they are offering.
The all-you-can-eat is a great example - except that they internet doesn't "run-out". So they want to artificially run out of "food".