Why did they take the dementors away in harry potter at the end?

at the end of hary potter and the prisoner of askaban, why do they take the dementors away, because even though harry knows sirius is innocent, everyone else thinks he still is guilty and isnt in askaban? so why would they stop the dementors from guarding the school now, because they still think that he wants to get into hogwarts to kill harry? i