In my state, Texas, the Penal Code specifies the exact wording of a written notice that must be posted in order for a business to prohibit concealed carry on its premises. Ironically enough, it's Penal Code section 30.06, so we refer to them as "30-06 signs". No joke.
Anyway, what it boils down to is that "gun buster" or "No Guns Allowed" signs mean absolutely nothing in Texas. With certain specified exceptions for things like bars and courthouses, the only thing that matters as far as signage is the 30.06. If it's not there in the EXACT form the law specifies, you have not been given notice and you may legally carry concealed, at least unless and until oral notice is given, and there are specific statutes about that also.
I don't know what state you're in, but if yours has a similarly specified sign in your statutes, and a business does not post it, you are probably good to go and I would strongly recommend you refrain from pointing it out to the business owner. You are not responsible for educating him on the law, and if you do, he's liable to put up a legal sign, and then you're really screwed (and that's when you write your letter). Or, you might also consider that he does in fact know what he's doing and he puts up the "gunbuster" knowing full well that he's still allowing concealed carry but he's falsely appeasing the anti's at the same time - the best of both worlds.
Just food for thought. Your state may recognize "gunbusters" or whatever, so this obviously wouldn't apply.