Bullshit. Prosecutors like their 100% conviction rates, to the point where they lie to judges, manipulate evidence, etc. http://www.propublica.org/article/who-polices-prosecutors-wh...

There are plenty of crimes that don't leave particularly useful evidence. Pointing a gun at someone is illegal, but how do you prove it happened? Same issue in many rape cases.

Cops in some areas also have a history of suppressing reporting and/or evidence to keep crime statistics down. http://en.wikipedia.org/wiki/Adrian_Schoolcraft