I agree that you wouldn't be at risk of getting caught carrying and having any type of legal battle based on that sign if you carry properly.
It's the "what if" I have to use it to defend myself that scares me!
If I"m in a non-posted place and defend myself from a deadly threat, then I'm pretty comfortable with the likely outcome.
If I'm at a place like ChuckECheese and have to do that, well, even if it's a totally legal and justifiable use of my firearm, I could still face a civil suit. Chuck E Cheese or AMC theaters or any of these moronic places that post, may just choose to come after me. That risk is what keeps me from even chancing it... So that + the fact that I won't support any business that chooses to disarm me keep me out.