I may be wrong, but I believe the main reason businesses, city parks, banks, etc. put up the gun buster signs is they are afraid of the legal ramifications of a civil lawsuit in the event of a shooting, justified or not. With the lawsuit happy and wacky lawyers of our society today, it's a real fear. Add to that some fear losing their $$$ more than their life and you get gun buster signs.
Sadly, they are not going away anytime soon. If we were having more physical bank and store hold-ups, and it was really getting into their pockets, it might change. But, if one innocent bystander got caught in friendly fire, it'd all be over with again.
My business debit card was hacked not long ago. The bank manager told me that most bank robberies were happening now days by means of ID theft. The threat$ they are feeling are not from the ones coming through their doors. They are only losing pennie$ there.