I don't agree with that policy, but they can set any such rules they like in that regard. Btw, is this a gunshop, range, sporting goods store, or other?
My point in asking is that the owner of a range may have somewhat more understandable reasons for this policy than the other places, maybe even based on unpleasant previous experiences. Also, owning a range entails carrying much more insurance, with no doubt restrictions that if not followed, can lead to the policy being cancelled abruptly.
I suppose if enough open carriers complained that this sign was going to make them shop/spend elsewhere, it could make the owner think twice.