If they are actively involved in an operation they would be fair game, nobody has claimed that. I provided a youtube video (edited post above), Obama is not claiming they have credible information as to their plot because he didn't have one, which is why they sent the information to DOJ months earlier.
Right now, under this theory - Trump can eliminate any Americans that go overseas for any reason or no reason.
If the U.S. has credible information that immediate threat is to be carried out than a target can be placed on them and there is no other choice, imo, in this instance they just didn't want the guy working for them and they simple assassinated the kid.