Sometimes, like the moments leading up to when a police officer decides to shoot someone, transparency is an unalloyed good. And especially lately, technology has progressed to a point that it makes this kind of transparency not just possible, but routine.

So it is in Rialto, Calif., where an entire police force is wearing so-called body-mounted cameras, no bigger than pagers, that record everything that transpires between officers and citizens. In the first year after the cameras' introduction, the use of force by officers...