Haha, boy have I heard that before ! They say that some day they'll get a detector, and thus don't want someone md'ing it before that. But you know full well they will never get around to getting a detector.
Also the song & dance of "No because of liability". Trust me: They could care less about "liability". The real reason is: They just don't want to be bothered.
It's not that they have any philosophical problems with metal detectors. But ... it's the psychology of the reason we all avoid solicitors of any type . Eg.: phone solicitors, door-to-door salesmen , junk mail that's selling something, etc... Even though it might be a GREAT PRODUCT, that does indeed "save you money", yet : The average person pays-no-attention, and simply doesn't want to be bothered with another thing to have to think about.