The "double action only" actually started with some revolvers, both in production as well as custom modifications. Several of the "hammerless" versions of S&W revolvers (actually internal hammers) are not capable of single action fire. Some police departments had in the past modified their duty revolvers to prevent manual cocking of the hammer, and the subsequent lighter trigger pull, for liability reasons. This is the same reason today, that so called "double action only semi-auto pistols" exist. The fear that a semi-auto pistol with a cocked hammer and light trigger is too dangerous for the police to handle.
LD