Comcast introduces new network management technique

I've received an email from Comcast describing their new "network congestion management" technique. They claim that it:

• would only affect less than 1% of all users on average and is "protocol-agnostic"
• is temporary and it has nothing to do with aggregate monthly data usage (it is dynamic and based on prevailing network conditions as well as very recent data usage)

Also, they provided some concrete numbers: "customers’ accounts must exceed a certain percentage of their upstream or downstream (both currently set at 70%) bandwidth for longer than a certain period of time, currently set at fifteen minutes."

This technique is in addition to and is distinct from the recently declared 250GB monthly traffic cap.

It sounds like Comcast will be temporarily increasing latency during a congestion for customers who are using more bandwidth than others at the moment. I wonder why can't they simply divide all the available bandwidth among all the users who request it? This way the slowdown would be spread among many customers and thus would only affect each one slightly instead of a few a lot.