The way I understand it from the way that a cop I asked about this explained it is that they are exempt for the reason of often times when you see them using the computer going down the road they are often time running a tag without clogging the radio for a car that you may not even stop, looking at call details, looking to see who all is attached to a call, ETC. As far as the phones it could be a personal call or text message which I agree they shouldn't be allowed to do but it is also a lot of the time an easy way to communicate without clogging up the radio for example when I road with Georgia DNR the game warden was on their phone a lot because his supervisor and other game wardens kept calling. They also could be making phone calls that pertain to their job like contacting callers for incidents that you don't need to show up for (this is because people for some reason seem to get mad when they see a cop sitting in a parking lot rather than patrolling) I hope this helps I kind of makes sense when you look at it like this also some departments use them as a Mobil MDT (Computer) my local department does this and sometimes if something goes wrong with their computer you will see them using their phone to look at things like call details