The mall is essentially private property and 'they' can make the rules.
I 'believe' (haven't looked it up yet) that the sign doesn't hold any weight unless your asked to leave the property because your concealed handgun was inadvertently 'discovered'.
A letter/call/email 'might' be good, but ultimately I believe it'd fall 'on deaf ears'.
If it makes them and others 'feel' safe, the sign'll stay. JMO.