Anecdote: I have a good friend of mine who would always come back home using the same route (same speed limit). One day they changed the speed limits. He got pulled over and ticketed by the sheriff. He got home, took his camcorder and recorded more signs being put in that day. He also caught the sheriff stop another car and letting the driver go with a warning. He asked the judge for a trial. At the trial date (the same day) the judge pulled him aside and told him the tape was inadmissible! Since it was his only evidence, he was convicted. This little anecdote showcases quite a few potential traffic court horrors:
-The cops never gave any warning of the speed limit change to residents (most people get on auto-pilot when they use the same route every day; there's no way they would notice a changed speed sign)
-Cops do not treat people equally
-Judges are far from objective in traffic court
-Decent citizens end up wasting half a day of work (huge cost) in order to fight a small fine. Most people would rather pay up rather than fight, even if they're innocent.
Seeing cops camped at 15 miles per hour roads ON THE WEEKEND is absolutely disgusting!
Your anecdote is describing systematic abuse. I certainly don't support that. It happens in the criminal justice system unfortunately; however, the answer is to fix the problem, not to quit enorcing laws.