Now that video cameras on cell phones are becoming common, we are seeing a lot of cases where police arrests and charges of disorderly conduct and resisting arrest are shown to be merely police covers for their own misconduct.
There is always a reason that a prosecutor does not believe he has a case he can win, and a combination of factual innocence and police misconduct is something that can lead him to drop charges.
I don't assume every person arrested is innocent, but I am realistic enough to know that false arrests are much more common than the police would like us to believe.