Mxsmanic wrote:

In that case, since pilots and ATC are also generally aware that people on
the ground are listening in, spotters and others listening to the
communications aren't breaking the law, either.

That case has been made, but has not stood up in court. If you are really
that interested, you could dig out the court records.

So is it legal for United to have a channel on the aircraft that allows
passengers to listen to ATC? *Why, or why not?

It is not legal in Germany, and, in fact, UA turns that channel off here.