Fans of The West Wing may remember this quote from Robert Jackson:
“The prosecutor has more control over life, liberty, and reputation than any other person in America. His discretion is tremendous. He can have citizens investigated and, if he is that kind of person, he can have this done to the tune of public statements and veiled or unveiled intimations. Or the prosecutor may choose a more subtle course and simply have a citizen’s friends interviewed. … While the prosecutor at his best is one of the most beneficent forces in our society, when he acts from malice or other base motives, he is one of the worst.
“Your positions are of such independence and importance that while you are being diligent, strict, and vigorous in law enforcement you can also afford to be just. Although the government technically loses its case, it has really won if justice has been done.”
http://www.roberthjackson.org/the-man/speeches-articles/spee...
Thus, they need no financial or social incentives; the work itself is their incenctive, much in the same way that tackling a hard programming program is incentive in itself for a talanted programmer.