I agree with this sign. I may be the only one here who disagrees with this law. I believe the owner of private property is lord and master of that property, and can determine what, or what is not allowed. If they post the above sign, you can still choose to enter their property AT YOUR OWN RISK. If you don't want to enter their property unarmed, you have the right to not enter.
That being said, "public" property should never be allowed to be "gun-free". This only applies to private property.