Yes, but because the radio used public airways, the courts found that the FCC can regulate them. It’s why you can’t swear on the radio (the FCC says so) despite free speech allowing swearing.

I'd be curious on how this philosophy translates to the internet today. There's lots of public infrastructure that we use, but it's hard to picture surfing the internet or watching Netflix as the same as broadcast television.

So, if the government had nationalized all paper products in the US, and then put strict content standards on newspapers, would the courts (and you) be like, "oh, well, that's not really a violation of the First Amendment, because the public owns the papers"?

I don’t know. I haven’t read the actual rulings from the courts. I’m just saying how things are.

Hardly. The Fairness Doctrine is exactly "the cure for bad speech is more speech".

https://www.britannica.com/topic/Fairness-Doctrine might be useful.

Compelled speech is not free speech.

Bzzt. It's not compelled speech. It's compelled airtime.