The argument is that it creates a derivative work. The right to create derivative works is separate to the right to distribute or reproduce the content, which is how Comcast is allowed to get the original website into customers' browsers.
Comcast are playing into this interpretation by adding their own license to the code they're adding.
It would open a whole can of worms. What about adding MPLS headers to packets, or performing MSS clamping? Or what about the numerous physical layer protocols that add error correction? Or the consumer routers that have parental filters, ad blocking, etc.?
Comcast are playing into this interpretation by adding their own license to the code they're adding.