The brakes on most patrol units are the same as the "regular" cars. The agencies don't take into account the extra 300+ lbs of electronics, trunk full of emergency hardware, and the weight of some of the cop's egos, (that was a joke, see how this works?), and the harsh life those cars endure day in, and day out.
There is a reason high performance cars have BIG brake rotors with multi piston calipers. It's not for bling, folks. But most agencies can't justify brake upgrades, especially when a brake job costs them less than a couple of hour's worth of city/county/state's mechanic's time, and a few bucks for new parts.