The signs are required for the enforcement of the gun free zone laws. These laws allow enhanced penalty for people caught in violation of DC gun law. The gun free zones are not intended to deter, reduce, or prevent crime, rather to increase sentencing.

From section: § 22-4502.01
For the purposes of this subsection, the term "appropriately identified" means that there is a sign that identifies the building or area as a gun free zone. (b) Any person illegally carrying a gun within a gun free zone shall be punished by a fine up to twice that otherwise authorized to be imposed, by a term of imprisonment up to twice that otherwise authorized to be imposed, or both.