I think they are in a particular position because a big part of their job is dealing with the members of the public at a time when they (the public not the police) are scared, vulnerable, maybe injured. This means that they have to be able to project a safe and reassuring image and I am not sure that, at present, facial tatts go along with this. In the future, or in a different culture, maybe they would....but right here and right now...no.