They do have a legal argument. They have one antenna for each user, so they claim they are just renting you the antenna, basically. Just as if you had rented the antenna and laid a long cable to their facility, then just used internet to get to the antenna instead of a cable. There are legal protections for encrypted stuff, heck the DMCA prevents bypassing just a copyright flag nowadays when backing up DVDs. But these are public TV channels anyone can watch with an antenna and the company seems to have legally constructed the same situation for its users.