If I’m baiting the arguments accurately, it was a unilateral executive thing, and only applied to officials of legitimate foreign governments. Which makes sense. Kill all the innocent civilians you want, but start killing the ones who actually start wars and they might come after you.
Even killing civilians on the other guys territory can create problems. I think the written policy was put in place as a function of reinforcing plausible deniability, in case a decision was ever made to use covert means to take out a foreign leader directly.