Because they will get completely slaughtered otherwise. Just like the brick and mortar music industry did.
AT&T and Verizon both own huge Cable TV segments and Apple has a right to come calling to get in with them. Apple has done very well for both of their bottom lines. If they start there the rest will cave.
Dont even get me started on the movie industry and their anti-piracy tactics. People want convenience and a reasonable price. Such a service could finally get them to move in that direction like the music industry did. People are going to buy 80 inch 4kx4k res 3D TVs so they can keep going to the theater and paying 10 bucks for popcorn etc?
How will Apple deliver their content to your home without an internet service provider? And you only support people or a company protecting their assets if it's Apple? How about Apple's DRM?