Peering is for peers. For backbone providers.
Comcast is not a peer. Comcast is an end user. Comcast should pay for both the inbound and outbound traffic onto a backbone.
Nowwww, this gets complicated as hell if Comcast owns or bought a backbone network.
I don't know. Maybe the old model just doesn't work any more, because in the old days "soruces" and "sinks" were spread out, now they're not, they're all segregated, network A is all sinks, network B is all source. And the idea that "source pays" seems kind of stupid. The siniks are the information consumers. Although I guess that provides no incentive for sources to get good network connections.
Maybe the "net source pays" should only apply if the traffic is traversing a network. If it's destination is on the given network, it should be "sink pays". If the source is on the given network, then it should be "source pays".
So you have a hosting plan, you are a source, you pay your hosting provider who pays their ISP. You have DSL or Cable, you are a sink and you pay your provider who pays for network. In between the two, anyone who accepts traffic that transits their network, well those peering points should obviously operate on some kind of "net source" manner, because that provides incentive for networks to build themselves out to reduce their "net source" charges.