It doesn't mean that some controlling autonomous will, independent of these desires and intentions, failed to operate, but that the net vector sum of motives and desires, as determined by various conditions, resulted in heavy drinking. Losing control simply means doing what one explicitly intended not to do, or doing what one would normally consider inappropriate. The captain very much wanted to reply in kind to the major's insult, but his strongly inculcated discipline in the face of such provocations won out. This means willpower is simply the strength of a particular desire, usually the desire not to give in to some other desire. It doesn't refer to some general capacity to quell or control impulses from some unmotivated standpoint.
Willpower refers to one side of the conflict of motive, usually the socially approved side.
Of course this voluntary decision, like the rest of Manny's behavior, was the result of various antecedent conditions, so the fact that the choice was voluntary shouldn't be construed as implying that it could have turned out otherwise, given these conditions. People make choices constantly, but they aren't the result of some contra-causal free will operating in some crucial sense independently of circumstances. To deserve a particular punishment, naturalistically, is to be liable for sanctions proportionate to the harmfulness of the behavior we wish to discourage. Regarding this example, some think that mere censuring is insufficient to deter such gross misbehavior on Clinton's or his successors' part, or more abstractly, to safeguard the dignity of the presidency. Therefore, Clinton deserves something harsher.