Mode S transponder display to ATC?

Michael Huber writes:

I'm going to violate my rule against replying to MXS and reply to
this, since I think it's an easy mistake to make. It isn't, as the ATC (and
any transmitting pilots) are fully aware that all pilots are listening in.
Thus, it is not an invasion of their privacy, since the transmitting party
is aware of these listeners.

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.

Further, it is actually beneficial and
intended for all pilots to listen, since listening to all transmissions can
enhance the pilots' situational awareness - they know what the other guy is
doing.

It is beneficial for people on the ground to listen in as well, as it improves
their situational awareness of the state of flights in the air.

So, the argument is that while only one plane is specifically
adressed, the intended recipients are really planes on the frequency.

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