I think this situation is somewhat different from your characterization. It isn't really a "public channel" that somehow belongs to the city/state. It is privat

I think this situation is somewhat different from your characterization. It isn't really a "public channel" that somehow belongs to the city/state. It is privately provided service that is just mandated as a side-effect of granting the private company a cable franchise. But I'm struggling to come up with a good analogy that involves private infrastructure that exists only due to government action with a 1st-amendment component.