Comcast won't throttle BitTorrent anymore, will throttle heavy downloaders

It always has confused me as to why ISPs and Universities have been so persistent about throttling protocols instead of just regulating bandwidth.

Quality is generally completely unaffected for other services as long as you don't saturate your upload bandwidth on DSL.

I always thought a nice system would let me pay by per-second bandwidth rather than by some contrived package combination of monthly cap and top speed.

e.g. $30/100KB/s, $40/250KB/s, etc.