Because laws like this never apply to emergency responders. You know, being highly trained and all. I wonder if us peasants could pay to get the same training they do. I'm sure part of the LEO training involves texting and driving.
One solution is for city's to pass regulations that have harsh penalties. I don't know how it'd work with the unions in bigger cities, but our town has a rule on the books that any city employee that is in an accident while in a city vehicle is immediately fired if it's found that they were texting while driving. It's not a legal penalty, but damn sure could ruin someones day. Our local police chief wasn't very happy when a few citizens showed him some photos of his officers texting while driving around our town square a few days after the rule went into effect.