Well, for international flights (and I think technically it's just flights INTO the U.S.), it's an FAA requirement that passengers stay in their ticketed cabin. Of course that doesn't really prevent J pax from wandering through Y in practice, but I don't think the spirit of that rule was really aimed at that (it was keeping pax away from the cockpit).

Domestically as there's no federal rule, it's really just a customer service issue. Yes, ideally Y pax should use the Y lavs and F use F, but on a single-aisle aircraft with no mid-cabin lavs that becomes very difficult to enforce when there are carts in the aisle. So what's the point of having a policy that's often difficult to reasonably enforce? Thus, no policy.

FWIW, on my last inbound DL int'l flight, one of the two lavs in the front of business class was inoperable, so there was no way I was waiting behind even one person in line, so I went back through Y and used the Y lavs at least once. Too bad.