Actually, the TSA
has come out with a new rule which does prohibit crossing cabins (either direction) ONLY on flights from outside the United States inbound to the US.
It is an absolute nightmare to enforce, because as the previous poster noted, it is not required on totally domestic flights or on international flight OUTBOUND from the US. Therefore, for once, when the passenger says "they let me do it on my last flight", they are probably correct.
Of course, no one believes you when you tell them this. They think you are being a rude, obnoxious ***hole. It's not just the lav that creates the problem, but when mom/dad are in the front and the kids are in the back, etc.
In my opinion, one of two things needs to happen. Either ditch the rule all together, or make it a rule for all flights. Inconsistency is what makes it awful. Failing that, at least a little publicity from TSA
about the rule would really help. But of course, that is "safety sensitive information" and the REASONS for the rule cannot possibly be discussed!
The views expressed are my own, and not necessarily those of my employer.