I don't think the term is exclusive to legal matters. Ie, I don't think it can only be used in situations with people committing treason on their country, ie real traitors.
It can also be used for a guy like Art Modell, who moves his team from one city to another, things like that. In everyday conversation.
So I don't generally have any problems with the use of traitor regarding John Kerry.
Regarding the specific case, John Kerry, I think there is no grounds whatsoever to even suggest Kerry is a traitor, even using the looser, everyday terminology.
However, no need for an outrage if Kerry is wrongly labelled a traitor, a few questions will sort things like that out. What are those claims based on and how does it make him a traitor.
No biggie IMO.
-Hedge