Who cares if the sign is legal? Concealed is concealed. Unless they know you're carrying whats the problem?
Now if you want to try therm on this I bet you'll have a problem whether the sign is legal or not. They'll say you knew the intent of the sign.
In my CWP class we were told to obey the sign even if it wasnt legal. You're best bet is to not do business with such businesses.
Do you seriously think that these businesses with the "illegal" signs would push the issue to charge you with a crime if your CC firearm is instrumental in ending a bad situation on their property?
I suggest that you study your local laws and get your legal advice from "competent legal counsel" who are familiar with the laws in the places you will be carrying. I strongly recommend against taking any "legal advice" from a LEO. Unless the said LEO is also an attorney, you may get "bad" advice.
Know the law and stay safe.
Here's the real (my) answer to any sign...legal or not if caught.....
"I'm terribly sorry, I did not see it, I will leave imediatly, BY"
Don't debate, argue or anything else.... if they want to discuss it with me they will have to while following me out the door.....
If they are calling the law then tell them you'll wait for them outside by your car since they don't want you inside and It would be trespassing for you to stay there. If the law is called I would not leave from the parking lot... in fact you might want to call the dispatcher (sheriff or city police) and let them know where you are and what you look like.....