It may not be free for long. Now that the signals are all digital, making them subscriber services only involves adding a set top box to your TV set. When a digital signal is encoded and you don’t have the correct decoding software, you don’t see anything.
The FCC would have to approve the change, but it could happen.
There is some history of OTA pay services. Back in the 80’s SelecTV provided movies through a scrambled analogue transmission.