A reasonable point. They fear:
1. The good guy may shoot an innocent. The risk analysis that the loss of a innocent to save 30 does touch them. This is because, there is a psychologicla bias about hurting an innocent even to save more innocents. Doesn't make rational sense but it is emotional.
2. The armed good guy may go running to or through the gun fight and get shot by the cops. Or shot when the cops arrive. Thus, 30 are dead but that's OK as we don't want the cops to get the liability of shooting an innocent.
Those arguments can be refuted but it helps to say that the good guys have a modicum of training.