Did anyone really believe that taking a job as a police officer somehow suppressed these people's natural desire to drive fast? Hell, that's probably what make them join the force in the first place. Sure, it's irresponsible and illegal for off-duty officers to speed, but it's also irresponsible and illegal for regular citizens to speed, and yet that doesn't seem to stop them from doing it. Why would you expect the cops to be any different? Do you hold them to a higher level of moral responsibility during their off time because of what they do for a living?
Being a police officer is a job. Unless you wanna have robots do it instead, people are going to do what people are going to do.