They are political appointees who serve at the pleasure of the President. They can be fire for lots of reasons or no reason at all.
"Justice Dept. officials gave conflicting answers when the House and Senate judiciary committees inquired into the dismissal of the federal prosecutors, who normally serve four-year terms and are replaced at the start of an administration."
This is not true or if it were had they fire all USA there would be no scandal. Stoopid scandal mongering out of whole cloth.