This seems unnecessarily judgmental. The problem here is that there is no standard practice. Some people flush the loo and their wash their hands before exiting the lavatory. Others wash their hands immediately after using the lavatory and then flush (in order to minimize transmission of their germs to the flush button or handle). Others never wash their hands at all. And of course a significant number of people don't flush either.

Short of trying to establish a standard practice and then policing it with in-lavatory attendants (!), nothing is going to change.