Historically, money flowed this way:
L3 <- Comcast for L3 to deliver Comcast's customers' traffic to the rest of the Internet Comcast <- Akamai to place equipment in Comcast's facilities Akamai <- Netflix to distribute Netflix's content
But recently L3 decided to get into the CDN business and signed an agreement with Netflix to serve their content--- but from L3's facilities. Their picture of the world:
L3 <- Comcast as before, but now there is some additional Netflix traffic which Comcast should pay for L3 <- Netflix
This makes it pretty obvious why Comcast is upset, and is claiming that the picture should now look like:
Netflix -> L3 -> Comcast for access to Comcast's customers
Reversing that arrow will not be easy; but, L3's assumptions of additional responsibilities (to Netflix) gives Comcast more leverage than if they were just another (very large!) customer.