Because it's only a matter of years until drones get small and stealthy enough that nobody is safe; exploding pagers are a clear first step in this direction.
As an American, I certainly hope they would fail. But do I think it's legal? Yes: it's a targeted strike on the leader of an enemy country they'd theoretically be at war with. Do I think it's wise? Well — no, Venezuela has a much smaller military, and assassinating the U.S. President would trigger a massive war that would devastate Venezuela for decades while modestly inconveniencing American taxpayers. But legal? Yes.
So then China could, at any point, call up the US president and say "Look there's a drone in the room with you right now. Shut down all your nuclear facilities or I hereby declare war and you're dead within 10 seconds." Then failing that they could hit the VP next, Secretary of state, etc etc.
Point being the idea of sticking with WW2 "rules" with current and future technology is laughably implausible.
And I guarantee you the citizens of Israel would NOT think it's perfectly legit, legal, and fair if Netenyahu got assassinated with a drone along with his military commanders.
If your opponent has way better weapons technology than you: well, it sucks to be on the losing side. But you are, and that's how war goes sometimes. That's what happened to Japan, and the Mayans. If one side has guns and the other has obsidian spears, there's no law that the side with guns has to drop them on the ground because it's unfair to the stone-age side.
there was 0 discussion about "legit, legal and fair". the only discussion that took place it's that security measures need to be improved to prevent things like this in future.
We've massed forces for an attack, attacked their ships, violated their airspace with combat aircraft (that's today), and extensively and publicly threatened them. They'd be in their legal rights to strike preemptively, including possibly a decapitation strike (this is why the Dubya administration kept repeating the term "preemptive strike", even though it was obviously nowhere near applying in the case of Iraq—it was a way of asserting its legal basis)
[edit] As thereisnospork points out in a sibling comment, however, this doesn't mean it'd be a good idea.
Not sure why you have doubts about this.
[0] Shouldn't - classic example of a tactical win being a strategic blunder. Killing the American president and would solidify American public support for the war - which would probably be undesirable in the balance.