Because there are some documented cases; because Kerry palled around with phony vets; because in a guerrilla war the enemy sets up villages to look like enemies, and innocent people get killed. There are plenty of instances that can be reinterpreted by Kerry's vision of things.
Why don't the Vietnamese themselves know of these "plenty of instances"? They were there, they talk, they would know. Why don't they?
Quite the opposite, every Vietnamese person I know - and that's substantially more than a handful - has the highest regard for U.S. troops on the Delta, whereas they absolutely despise the French and Japanese, and their behavior there is recalled as a horrorshow.
It's like the alleged Vietnamese "witness" to the Bob Kerry incident in Thanh Phong: the place was a known VC hamlet, the villagers all hardcore VC, and the "witness" herself was the wife of an admitted and longstanding VC operative (who presumably was out on a mission, since he all the other males were absent that night).
Nope, Kerry was and is a phan boi. Period.