This is one of those normative signs that reminds you that every rule has a history. It’s instructive to consider just what the problem was, here, that inspired someone to computer-generate a sign to post on a mirror.
The picture was taken at the nature center in Holliday Park in Indianapolis, IN. If I had to guess, I’d say that the problem was hikers washing their muddy boots, clogging the drain. In other contexts, such a sign might be de facto discriminatory, because some people (homeless people) don’t have anywhere other than public washrooms to do that sort of thing.
Thanks to Abe Schwab for sending the pic.