Sovereign nations should lookout for their own best interests. In the event that the US elects John Kerry (and remember we DID do Clinton TWICE so it could happen) then they have a responsibility to defend their own citizens.
You are correct in that individual nations should look after their own security interests. However most know that for 50+ years, Canada,Japan,South Korea and most of west europe have spent a small % of their g.n.p. on defence knowing that they have had the U.S. to foot the bill.