I'm unfamiliar with ISP scale networks - where is the actual bottleneck for them? One of the large ISPs in the UK is Virgin, and they have their own fibre network. They also throttle Youtube.
Is there only so much data that can go down a fibre line? Is it the routers (I'm not sure if calling them routers is correct?) that lie between the main backbone and the network connecting a street/apartment? Is it the hardware in a handful of large data-centers?