This is Maryland. What you are speaking of does apply in other jurisdictions. In Maryland the MSP is purposely leaving the "except where prohibited by law" part off of restrictions just in case they decide they want use it against you. Remember where you are. Remember MSP testified to basically exactly what I am saying when someone (a member here) tried to get the restriction modified to include the "by law" part.
I am not saying you will be prosecuted. I am not saying you will be arrested. I am not saying signs carry the force of law in MD. I am not saying I believe it to be a violation of restrictions personally. I am simply saying MSP worded it the way they did to be able to use it against you if they want to. Never underestimate the intent of those who wish to disarm you.
They can't use it against you unless they have a charging document, and charging documents and codes are created by the passing of laws by the Legislature. The charging codes are what prevent them from "using it against you if they want to."