Well, it isn't entirely true that he's never been accused of war crimes. A hack writer for the Boston Globe did it way back in 1996. And then the accusation was retracted.

From the incomparable Daily Howler:

"For starters, let’s consider those war crimes. One week before Election Day 1996 (Kerry won re-election over Massachusetts Governor William Weld), the Globe’s David Warsh published a startling 2100-word piece about Kerry. In it, Warch suggested that Kerry had committed a war crime in Vietnam, in the action for which he was awarded the Silver Star. (And yes, Warsh used the term “war crime.”) Almost instantly, Warsh’s factual presentation was shown to be bogus, and the crackpot columnist later said that he shouldn’t have used the inflammatory term “war crime.” The paper’s ombudsman, Mark Jurkowitz, said that the story should never have run. “[T]he issue is whether the column belonged in the Globe. The answer is no,” the ombudsman wrote. But then, the Globe had been messing with Kerry all fall. Three weeks before Warsh’s strange report, the Globe had published the 4000-word profile which misled Keller six years later. As Kerry tried to win re-election, the Globe was printing silly stuff about his “home movies”—and the nastiest of all possible attacks. And both of these stories were bogus."