When Howard Dean speaks, he’s speaking as the chairman of the Democratic Party. The Democrats pay him. If there are Democrats who don’t like what Dean says, then they can stop donating to the party (they ought to have sense enough to keep their disagreements private, but that's tangential).
When Dick Durbin speaks, he’s representing the people of Illinois, to whom he will answer when he’s up for re-election.
When Karl Rove speaks, he speaks as an official with the White House. He is no longer simply an evil-genius-political-operative extraordinaire, he's also responsible for shaping policy and stuff, due to his not-so-recent promotion. The only person he’s accountable to is the President of the United States, who has no intention of asking Rove to apologize.
Karl Rove is paid by each and every American taxpayer. He represents all of us.