Slightly off topic: The FAs do not seem to enforce the rule when the business class lavs are at the rear of business. How often have you seen the FAs enforce the rules in that case?

On a 767, the lines for the business class lavs are clogged with folks from economy class. And yes, the benefits of an upgraded cabin are supposed to include shorter lav lines and cleaner lavs. There is a loss of benefit to business class passengers when economy passengers use the wrong lav.