Teams are surprisingly consistent in choosing who will be good at enabling them to be effective; they know who will get the best out of them. It is also often very different from the person that management tends to select for the role! Given that the team is not accountable to the ScrumMaster and, if anything the ScrumMaster is accountable to the team, it makes a certain amount of sense for the ScrumMaster to be chosen by the team.
