I'm going to play devil's advocate here, but there might be legitimate technical reasons - streaming from your ISP's network may actually be easier on the netwo

I'm going to play devil's advocate here, but there might be legitimate technical reasons - streaming from your ISP's network may actually be easier on the network (it doesn't require inter-network hops, or maybe, since they control the content, they can cache it closer to the edge, etc.). There may also be financial reasons - if people are paying them for a separate streaming service, then it's easier to justify investments in infrastructure to reduce the congestion from that service. That assumes that the congestion isn't at the edge/CO/etc. If it is, then I don't know how to justify it, except as a way to get people using their service, which is obviously part of it, either way.