The rationale is that as there are more cyclists, motorists realize that they must adapt to their presence and (in effect) drive more safely (relative to the cyclists, anyway).
But when there are a lot of bicyclists on the road, according to this theory, drivers take notice. They become more attentive, slow down, pass more cautiously, double-check their blind spots, expect the unexpected. They sense that the road has become a more complicated place, and adjust their behavior accordingly. As a result, the road becomes safer, presumably for everyone.