Self-interest, public interest, and the efforts to influence public policy

Why is it reasonable to believe that politics/policy-making are exempt from self-interested behaviour, while it is 'obvious' that in one's business, personal, occupational and even spiritual life that people act in a self-interested fashion?

It is common for lobbyists to be charged with self-interest—as if the other players do not pursue their self-interest. For example, Donald Savoie, the Canada Research Chair in Public Administration and Governance at the Université de Moncton, has argued that "Power and influence in Ottawa is centered around court government, around the Prime Minister, around lobbyists, around spin doctors and our democracy has been bastardized, if...