You can't disarm criminals with laws. You might be able to do it with metal-detectors and bullet-proof glass. The last time I was in the local police station, I entered a narrow hallway, which was no doubt constructed of bullet-proof materials. The hallway ended in a very comfortable looking waiting area, with seating, tables, and reading material. The "receptionist" police officer was seated behind what appeared to be a window made of glass at least an inch thick. I spoke to him over an intercom.
There was no way that a suspected bad guy was going to be able to hurt anybody except other visitors.
After I showed my picture ID and explained that I was there to get my Florida CCW fingerprint card filled out, a door opened up and a polite officer escorted me into the controlled area of the station. I was not searched nor did there appear to be any metal detector. I was simply presumed to be law-abiding.
It's really a PR issue. There is no need for a law abiding citizen to have a gun in a police station. If you let people bring in guns, some criminal shoots the place up, you look like an idiot. Not only that, but you open yourself up to the likelihood of an overreaction.