If Cops accept cards through an organized and supported system, seems to indicate they would take bribes just as easily.
In places where cash bribes are common, police will pull people over -- even if they've done nothing wrong -- in order to solicit a bribe.
That sort of thing isn't possible with the "courtesy cards", so police stops are more likely to be for legitimate reasons (or at least no more or less legitimate than in an ideal system), but people who shouldn't get away with things often will.