I don't recall any advertising and marketing for J that mention J toilets are reserved for J passengers.
When did J passengers get the expectations that waiting for toilets is not a J thing?
If the Y passengers are told at the beginning that the main cabin toilets are at the rear and then later they come to the front and the CSM does not stop them, then that can be assumed to be tacit permission to use the front toilets.
As I said before I have used F,W,Y toilets when J (and J when Y). So I am very happy for Y passengers to come up to use J toilets. Did not get a finger wag from CSM ot cabin crew either.
I suspect that this continues because very few actually complain about this and also the PB non-issue. QF has been known to change dress code on the basis of customer feedback about the lounges. but the non action in this matter maybe points to the fact that very few see it as an issue.