I believe every president has done these firings at the start of there terms, just as they place in new department heads. However there is a bit more involved in these firings then a simple changing of the guards.

Here is an interview of David Iglesias on CBS. I hate Katie Couric, but atleast you can get a view of one attorney involved.
Zimzum Reviewed by Zimzum on . Firing of the U.S. Attorneys Does that mean there will be a problem with the fed's prosecuting people for marijuana offenses? Rating: 5