HIV transmission happens when infected blood, semen, vaginal fluid or breast milk enters another's body. In the case of scratching or nail biting, those are not very risky situations. It would be very hard for HIV to be transmitted in such a case unless there was a lot of blood present and an easy way for exchange to happen. My guess is that by giving the police officers chemoprophylaxis they are operating under the "better safe than sorry" principle and not because it is a high risk situation.