A good manager does more than just squash squabbles amongst developers he or she helps to convey business requirements to development, inspire the team, and meet other non-dev-y needs in the hope that a good manager helps the team accomplish more than it could without said manager, hence the salary. Or that's how I've always justified their salaries. There's also the experience that they often bring to the table that usually warrants a higher wage.
Management is difficult and effective managers are rare. Really good ones can burn out quickly. Employees don't always appreciate how stressful the responsibility, long hours, and drudgery can be. My $0.02.