[Spytap]

Since I only have experience working with American broadcaster, my answer will come from that POV.

Realistically, neither of the two biggest reasons involves the protection of the dwindling home video market. Broadcasters make a lot of money off of their local affiliates (who pay them for the privilege of running their programming) and cable providers (who pay a subscriber fee for the ability to carry that network.) Each of these existing revenue pipelines brings in a lot of money - orders of magnitude more than any torrent site would. Now you may say "why would a torrent site disrupt their existing streams?" and the answer is that while it wouldn't, those existing streams are very protective of their domain, and would likely consider it an immediate breach of their exclusivity, and in the long term would use the existence of a "competing" product to lower their fees on renegotiation. So it's a lose/lose for any network or cable providers at the moment at least in the US.

That being said, it's their job to have their business model compete in the modern world - it's not the job of the modern world to adapt to their business model. Either they'll figure out a method to do so, or they'll die in 6-8 years and we'll build something better.