The only way I can see of justifying them would be to consider them to be precise military strikes that are part of a larger war effort, but if that's what they are, then "assassinations" are a poor name for them.
This is just one example of the inevitable lawlessness that the concept "war on terror" leads to. A bomb, H bomb, Minuteman / The names get more attractive / The decisions are made by NATO / The press call it British opinion -- The Three Johns