Good encounter! However, why do you want to praise his actions when he did exactly what he was supposed to do?
IMO, I would definitely write the Department but emphasize that he did the correct thing. Don't make it seem like he went above and beyond the call of duty in resolving the situation.
I agree... while it is good that he did not abuse your rights, should we give them praise like its something special.
Kind of the same way when a mechanic does a good job fixing your car, or when you have someone else perform a service for you and they do it in a good professional way do you not say thank you and then if you felt like you received good service recommend them to others. Why do you feel that a Police Officer, who the OP felt did them a good service should not be praised in the same way. Or is it because it was a Police Officer you think they shouldn't be thanked publicly.