Then why have the sign?
red
Because sometimes expectations aren’t clear to all parents unless they are reminded of them.
A parent who truly disagrees with a sign like this is likely to ignore it - and I think that’s fair enough if they really do disagree with it. But far more often, it’s simply a matter of acting without thinking too much about it. Explaining the thinking behind a rule (or a guideline or whatever it is you want to call it) is something schools should do whenever they can.