Guilt by Association?

Why is it that those who are related to the accused or are unfortunate enough to be dragged into a police investigation in a capital crime feel the heat? Why should they be blamed for assisting the police with their enquiries? And what about cops who bungle cases--should they be promoted or asked to leave the force? After all, the Portuguese cop who headed the enquiry into the disappearance of Madeleine McCann lost his job when his investigation failed. Shouldn't the same rules apply to Indian cops also?