In part because it's so bereft of any objective definition beyond what the person complaining about "speeders" considers to be "speeding" – usually defined by him as anything faster than whatever speed he is comfortable driving.

Well, there is the legal definition. Which is objective – in a completely arbitrary way.   

"Speeding" being defined under the law as driving any faster than whatever the arbitrary number on the sign says is the maximum permissible speed. Many people mistakenly equate this with the maximum safe speed and consider that anyone who drives any faster is driving "unsafely," by definition.

Such "speeders" are more than merely deserving of punishment. They are bad people.

Reckless people.