Why is presidents salary so low?

A half-million dollars a year ($400,000 actual salary, another nontaxable $100,000 for travel, plus a $50,000 expense account and $19,000 personal entertainment account ... not including state dinners, which the government pays for, and whatever living in the White House with access to all its facilities rent-free is worth) is low? We are clearly in two very different socioeconomic classes.

Now, it is true that people who have held the office tend to be extremely well-off to start with, and for some of them it does represent a "pay cut" from what they were earning as CEO/chairman of the board/whatever in the private sector. Still, it's hardly "low".