Why Didn't the Generals Resign?

Thomas Lipscomb asks a question and gives a history lesson to some dissenting ex-military men:

At least Wesley Clark got himself fired and summarily retired as NATO commander in comparative disgrace for submarining the Balkans policies of his Oxford classmate President Bill Clinton and his defense secretary, William Cohen. Gen. Billy Mitchell is regarded by many as having saved American military aviation by accepting a court-martial and resigning from the service he loved because of his differences in policy with the federal government...

But if Generals Gregory Newbold, John Batiste, Zinni and others have believed Rumsfeld's policies have been so dire that they are calling for his resignation, their opinions would have carried far more weight if they had stated them at some personal cost to themselves while on active service by resigning in protest. That action might have also carried some evidence of the courage Americans expect of the highest ranking officers of its uniformed services.

Could be they are campaigning for a Democratic nomination?