It's a TSA rule - hence the dumbness of it. As mentioned, it only applies to international flights, because in TSA logic, terrorists only attack international flights via the F or C cabin. Of course no one ever pointed out to the TSA that most F and C cabins on wide-bodies have lavs at the rear as well as the front.

No matter - from a service perspective, the F lavs should be for F customers only. I don't get upgraded or buy F all the time, and while sitting in Y I've never felt the need to walk forward to use the F lav or store my bags in the F overheads. It puzzles me why others seem to think it's OK to do that.