Due to technical reasons, it is possible to have many, many more cable or satellite channels than over the air broadcast channels.

The FCC is allowed to put “reasonable” restrictions on the content if the over the air channels because of that scarcity.

The default position in the US is that both cable and broadcast have a 1st amendment right to say whatever they want. The FCC is only allowed to infringe on that right for the broadcasters, because they are consuming a scarce public resource.