Just about every USA bound (or exiting) flight (let alone just about every QF flight I have been on) have had the announcements that you are to use the lavs in "your ticketed cabin" (UA love to use that phrase ). Now QF/NZ/UA (who I have flown on over the years) have all claimed it to be either a TSA rule or FAA (or whatever) but it's always been clear. QF also of course basically tell people to use their "own" (class) lavs since well, J pax don't want or need Y in their lavs and so on.

I would be very surprised if AA had a formal policy regarding this. sounds like someone just went forward and a FA didn't make a big deal about it so suddenly it's OK.

Not in my experience.

Of course there's announced policies and what goes on in flight.