Legal Question in Civil Litigation in California

It's called being hassled by the police,

It wouldn't be against the law if the police had good reason to detain you. Prosecutors may have decided later that the police were wrong, or may even have believed they were right but that the case could not be proven beyond a reasonable doubt. Or they may have believed the case was winnable but that it wasn't worth pursuing (prosecutors do not have enough resources to pursue every viable case). It's even possible that you will be charged in the future even though it hasn't happened yet. There are other legitimate reasons for such a detention without subsequent charges.

Of course, it's also possible that the police detained you illegally. Such things do happen. But the fact that you haven't been charged is not enough to show that this is what happened to you.

It's called a false imprisonment and violation of your civil rights. Let me know if you want to sue them.