I would say it depends on where the xfinity servers are located. If comcast has the servers on its own network, or has a peering arrangement with the network which xfinity servers are located, it is possible that they are not paying as much for the bandwidth when compared to the bandwidth that is used by their users going to netflix servers. If netflix wants net neutrality on this issue, then they can offer to pay for the bandwidth that connects netflix to comcast.
This reminds me of the old AOL network where content providers paid AOL to be connected to the "premium" access network that enabled AOL users better access. At that time AOL could dictate the terms since it was one of the few games in town which had ALOT of users that the content providers were eager to get their hands on.
A long time ago a local university had really bad connectivity to the internet (it had to go to the "main parent" campus then back out the the internet to reach an ISP that literally was half a mile away. The university and the ISP decided that a "mutual peering" arrangement was beneficial to both since the ISP had more than 70% of the local market at the time and most of the local market communicated in some form with the university servers....
Someone at netflix should have thought of that (peering with comcast/charter/etc) with a "dont charge your users for the cap when connecting to us via the peering connection" deal...
Hulu should also consider that as well....