It's a technical but important distinction (not to imply there's no hypocrisy here, just noting that they couldn't smack Fox if they wanted to).

Note Carr's constant references to the "public good". This concept applies only to those who use the public airwaves.

Carr only has authority over broadcasting. ABC/CBS/NBC/NPR have broadcasting properties and affiliates so he can threaten action on them. Fox News doesn't broadcast, they only deliver cable and streaming channels, so the FCC can't touch them even if they wanted.