When you do NOT allow guns, the only people you stop from carrying them are the good guys.  Do you think that a criminal enters an area with a hand gun, sees a sign and says "oh...I better not take this gun that I purchased illegally and am carrying concealed illegally and intend to rob someone with into this area! It's a gun free zone!" 

No.  They LOOK for these areas because they KNOW that a law abiding citizen will NOT have a firearm to defend themselves with!