People have a civic duty to cooperate with police to make their communities safer, true.
The problem is that the police abuse this relationship to rush investgations and slap names on casefiles. If you say you were there, you become a suspect and if it goes to trial then your life literally falls into the hands of the most simpleminded people that voir dire couldn't eliminate, or a single "impartial" individual.
Or, a simpler (real!) case- suppose you drive away from a bar having had too much to drink. You realize your mistake, pull over safely and pass out behind the wheel. A carjacker approaches you and steals your car at gunpoint. You go crying to the police.
Soon as you tell them what you were even doing there, you're facing DUI charges yourself. You become the bad guy.
Even if your car is recovered, you won't be driving it anytime soon. You'll lose your job if your employer finds out . You'll spend thousands on legal defense. All told you end up being punished more severely than the actual criminal-- because even if caught, he has nothing to lose.
And this was all brought about because you freely admitted to committing a crime yourself.