See What Conflict of Interest? How Power Blinds Us to Our Flaws from The Wall Street Journal. As the article says "...prohibitions on insider trading generally don't apply to Congress." And they only have to report their trades once a year. They can make trades based on information most people don't have. Here are some exerpts from the article:
""Power makes people feel both psychologically invincible and psychologically invisible," adds Adam Galinsky, a professor of organizational behavior at Northwestern University's business school.
Power, explains Prof. Galinsky, focuses people on their own internal goals—blinding them, in the process, to how others may view them. In Plato's "Republic," Socrates invokes the myth of the ring of Gyges, which conferred upon its wearer the power of being invisible to others. If we wear such a ring at will, Socrates says, "No man would keep his hands off what was not his own when he could safely take what he liked."
Being in a position of power also may make people feel that they can do no wrong. In recent experiments, Dana Carney, a psychologist at Columbia University's business school, has found that acquiring power makes people more comfortable committing acts they might otherwise be reluctant to commit, like lying or cheating. As people rise to a position of power, she has shown, their bodies generate more testosterone, a hormone associated with aggression and risk-taking, and less cortisol, a chemical that the body generates in response to stress.
"Having power changes you physiologically, reducing your body's internal feedback that tells you which actions are good or bad," says Prof. Carney. "Power temporarily intoxicates you.""