Admittedly, it's not really physically possible to catch up to someone traveling at 90 mph when you're limited to 65, however they have lights and sirens that, according to law, they are required to use while responding to an emergency or incident, which broadly covers speeding.
According to the original article, more than half of all incidents occurred outside of the officers jurisdictions. Miami-Dade limits officers to 20 mph above the speed limit at all times, even while enroute to an emergency (270 cops were found going over 90, 70 being the highest speed limit in Miami-Dade). FHP limits officers to 15 above the posted limit, but does allow excesses if the situation requires it, but only if the environment allows it and it is safe to do so, but earns the prize for the fastest speed of 129.8 mph.
http://databases.sun-sentinel.com/news/broward/ftlaudCopSpee...