Notice that "American" was missing.
Rather like having Kenya attack Tanzania and the US bomb one side. Of course the other is in its rights to effectively say that the US declared war on it and threaten retaliation.
No point being upset at the threats of retaliation, because it is retaliation. It's unreasonable to attack somebody and think that they'd say, "Right-o, my good man, you killed a few of us when we hadn't done you any harm, but that's okay because you're just swell. Bombs away, kill more of us because, gee, what a guy!"
Suddenly I expect Obama to say, "Smoke me a kipper, I'll be back for breakfast."
And I suddenly hear crickets instead of, "If you kill one, you create 10 more."
I assume the CiC thought this through and realized that by the attack *before* we were attacked we're potentially a target, in a rather more direct way than many said the US was a target on 9/11 and with the Cole, Khobar towers, Kenya bombing, etc., etc.
The only problem is the whole "khalifa" thing, because many scholars said that only a khalifa could properly declare jihad and ruled the "emir" bin Ladin out of order. But these fruitcakes declared themselves the khalifate--also mostly ignored. If they then declare jihad on the US, that may lead those who want to have "proper jihad" against the US to agree that the IS is the khalifate, or make those few who accepted the declaration of "khalifaticity" as valid to think, "Crap, now there's a jihad and I have a moral duty to uphold it." It's easy for any terrorist to now attack US people or property, as he would have anyway, and claim this as justification.