Without the service L3 provides, Comcast wouldn't be able to (as efficiently) provide the service to their customers. In a sense, Comcast should pay L3 for their access to the content.
Um, didn't someone upthread point out that Level3 doesn't actually carry any Netflix content?
You can try to package it any other way you like, but that's what you are suggesting: The only reason Comcast is in position to try this gambit is because there are alternatives (albeit very few) to backbone providers, and major content providers can be bullied directly, but most Comcast customers do not have alternatives to Comcast.
If anything, that Comcast even dares ask for this should be evidence that they are abusing a monopoly situation, and ought to be a trigger for massive reform - in a free market, they'd get told to take a hike.