Because that is the conclusion they drew. Why do you say the argument is ad hominem? Are you saying that any line of reasoning that leads to the conclusion that someone is likely to betray his country is by definition an ad hominem argument? Lets say that, in addition to the facts cited by moveon, we learned that General Patraeus would make millions of dollars and get to sleep with Halle Barry if he made the case for continuing the surge. Would you still say that it is ad hominem to suggest that the might give a false report?