The Question is Why:
I've been reading a lot about the U.S. Attorney purge story recently, and I think it's helpful to be clear about where the story is: In my view, the key question here is simply why the U.S. Attorneys were forced to resign. No one questions the President's legal authority to force the U.S. Attorneys to resign. No one suggests that forcing them to resign was illegal or is demanding their reinstatement. Rather, the question is simply why these U.S. Attorneys were pushed out.

  Some readers may ask, if no laws were broken, why should Congress be able to investigate what happened? I think the reason is that while the President can fire U.S. Attorneys without cause, Congress has a lot of control over what happens next. Until 2006, a replacement interim U.S. Attorney was permitted to serve only up to 120 days. After that, Congress required the district court to name a replacement interim U.S. Attorney. See 28 U.S.C. 546 (pre-2006 version). In 2006, Congress amended this law in the legislation generally known as the Patriot Act reauthorization law (not the actual Patriot Act of 2001, but the reauthorization in 2006). As amended in Section 502 of that law, a person appointed as U.S. Attorney on an interim basis "may serve until the qualification of a United States Attorney" through the usual Senate confirmation process.

  As I see it, the key point here is that Congress has statutory control over the rules governing how open U.S. Attorney spots are filled. Although firing the U.S. Attorneys was not a crime, Congress has a legitimate oversight interest in learning about how the Executive has used the power Congress has granted it. Why the U.S. Attorneys were fired seems to be directly relevant to that; Congress has a good reason to want to know if loosening the rules led to replacements of U.S. Attorneys for reasons that are not based on legitimate law enforcement concerns.

  Of course, the fact that Congress has a legitimate interest does not answer everything; there are lots of competing legitimate interests here, and that's only one among them. But I do think it's helpful to see that the issue here is why the U.S. Attorneys were fired rather than the legality of the firing itself.