If that's what's at issue in my case, why wouldn't I see some error message to that effect? Simply having the player window do nothing is a rather strange (not to mention unhelpful) way of enforcing HDCP. If the DVD Player app (also bundled w/El Capitan) is working, why not iTunes?? Just doesn't make sense!
Its a widely known issue that has been around for a very, very long time. I doubt anyone has figured out why this happens.