"he's being disingenuous, not dihonest."
That's almost splitting hairs.
A real tour of duty would be for a fixed amount of time, set by the military, and then completed. Then completing another set amount of time would qualify him for serving two terms, hence tour(s). I fail to see where Kerry served two tours, which would qualify him for using the plural of tour.
I don't disagree with you. I'm just saying he will defend his claim by saying he was there on two seperate occasions.