Yeah, let's start kicking in the door of a house that got robbed, the owner won't at all be jumpy and assume it's the robbers again, honest!
The informant provides the opportunity for this sort of thing, not any evidence of crime. The police do not care at all if what the informer says might have any basis in truth or even “honest mistake.” To the Law it is an opportunity to do the things they enjoy and have been trained to do.