In my experience the two main limits to availability of live ATC in the form of Channel 9 are the very few domestic aircraft having the capability and, for those international craft that are Ch. 9 enabled, that direct radar monitoring of flights doesn't occur more than about 200-250 miles from land-based radar.
That over-ocean flight is monitored by non-radar means with HF and ACARS communication is likely the reason that most UA pilots switch off channel 9 or replace it with some other audio; and some may forget to switch back when their over-land segment resumes.