Why is "have you informed the police" wrong?
Your sentence is a different tense, namely present perfect (have + participle). Asked here is simple past, though. Your sentence would be translated as (if I'm not mistaken) "Har du informeret politiet?"
Why is "You informed the police?" not accepted?
I agree this should be accepted. It is proper English grammar and it means the same exact thing.
'Have you informed the police' means the same as 'did you inform the police'!