I don't think there's much to the story about the US Attorneys being fired—it's a political appointment, and Bush could legally fire all of them if he wanted. There are a bunch of other things to get mad at President Bush for. But here's one of the complaints that were forwarded to the President about one of the prosecutors(Emphasis added):

Representative Lamar Smith, Republican of Texas, also asked a White House official to explain why prosecutors were pursuing charges against illegal immigrants only if they had been counted entering the country illegally eight or more times.‘Loyalty’ to Bush and Gonzales Was Factor in Prosecutors’ Firings, E-Mail Shows By David Johnston And Eric Lipton,New York Times, March 14, 2007