Because transit providers are actually providing a service to Netflix. They allow them to dump all the traffic in one location and then the transit provider will build out a network to take care of moving the traffic to anywhere in the globe where there's an ISP requesting it. The transit provider will then not pay the ISP to take the traffic and deliver it to their requesting user. When Netflix connects directly to Comcast that's not what's happening, Netflix has to get the content to Comcast's network by itself and the Comcast network is already paid for by the user. So if Netflix pays Comcast then Comcast is getting paid to create the network by both the user and Netflix.