Traditionally in the US politicians, especially presidents, try to cultivate an image of familiarity with the people. Hence they are often referred to by last name only, or in some cases by nickname (Dubya), diminutive first name (Jimmy, Bill, Dick), or initials (JFK, LBJ, FDR). "Mister" is used almost exclusively by journalists in my experience.
In a sense, this is entirely fitting and proper since the president does work for, and is ultimately responsible to, the citizens, and it is the person of superior status who has the prerogative to set the level of formality of address.