There is some very illuminating comments by David;

DI:  It is.  Right, you're appointed by the president, you're confirmed by the Senate, you serve typically anywhere from 4-7 years.  Some people leave earlier, some people get fired, although that's relatively unusual.

And what makes this scandal so different is that large number forced to resign on the same day with no real justification, with no legal basis for that.

and

I knew you could be fired for getting involved in political matters as a U.S. attorney, but I never thought I would be fired for not getting involved in Partisan political activities.