Technically, calling the soldiers who confronted Rumsfeld today "disgruntled" is a correct use of the word. According to my Yahoo! Reference, it means "to cause unhappiness by failing to satisfy the hopes, desires, or expectations of."
In terms of real-world communication, however, there's a full five-piece set of baggage attached to the word. In general, we do not use the term "disgruntled employee" to describe someone who complains to HR or lays out some hard truths for their boss. No, we save that term for the employee who keys the boss's car or slashes his tires in the parking lot after work. Or we use it to describe former employees who take out their frustration by returning to work with a shotgun.
Disgruntled employees are those who take out their anger with excessive inappropriateness. The subtext to the subtext is that their disgruntled-ness is misplaced or unfounded.
Does any of that apply here? Exactly what information does the AP (no point in holding Fox to any standard) have to support the use of this inflammatory term?