The GOP story line says that the fired US Attorneys are "political" appointees and serve at the pleasure of the President. So who cares if they were fired for not doing their President's political will? Speaker of the House Nancy Pelosi puts one of the most basic tenets of our independent judicial system into perspective:
"That prosecutor must be objective and what I worry about most of all in this is the chilling effect this has on objectivity of the American U.S. attorney who is the main prosecutor for the federal government of big cases under federal law."
Objectivity is the core of our legal system. While it's true that these attorneys are appointed by the President (with Senate approval, normally, but that's another issue), once they take their oath of office they are compelled to be objective and not be swayed by politics, but by evidence. That's why it's illegal for any politician to try and influence a US Attorney's investigation, but I guess that's news to some Republicans.