This is a ploy. No ISP uses more than 10% of their current bandwidth, if they wanted to you could throttle it up. They are just scared that their infrastructure won't hold with the market as it grows. This also gives them an excuse to charge more for their services so they can 'expand' their infrastructure.
Take a look at Japan, in their major cities the whole place is WiFi and they have a fiber back bone running to each major building. So, when you rent an apartment in say Tokyo, the building is already jacked into the fiber backbone and they run that fiber to the tenant. So, you have fiber in your house. Not so much of that in rural Japan, but in the major cities it is very common. They had to rebuild their infrastructure after WWII so it is more modern than ours in our major cities in the US.
They have been trying to charge more and tax for this stuff for a long time. They just haven't found their angle yet. Companies really want to charge you every time you use your email. They would do it too, if they could get away with it. This is there way to charge more.