In a way, I agree with you on this point. In fact, in my ideal world, all weapons would be banned entirely. If nations had a dispute, their top leaders would meet for a winner-take-all checkers tournament.
Unfortunately, humans don't trust each other - and that goes double for other human governments. We can be a very ruthless and predatory species, and because of this, we think everybody else is like that, too.
We don't want to appear weak before our adversaries, so we try to put forth this "tough" image and carry a big stick. "Speak softly and carry a big stick" is the philosophy which seems to give us our perspective on the world and how to carry out foreign policy. It's just that now, our "big stick" has a lot more firepower than it did in Teddy Roosevelt's time.
I've encountered a great many people in my life who have a certain warlike or militaristic attitude about US politics and foreign policy. They're generally ardent patriots, and while I don't find any fault in that on its face, their viewpoint is generally based upon the notion that most Americans are naive and gullible about the outside world. The idea is that most of us are protected, sheltered, and coddled in a certain idyllic, luxurious "bubble" full of good-hearted, compassionate, generous, wonderful people who need to be protected from all the evil and horrible bad things in the outside world coming to get us.
It doesn't really matter if it's true or not. A lot of people tend to think along these lines. If there are Americans of a more liberal bent who oppose US militarism, it's only because they're naive and simply don't have any conception or clue about just how dangerous and evil these other "enemy" countries are. They don't understand how they think or how they're cooking up all kinds of plots and conspiracies to destroy America and our way of life.
I'll admit that this kind of talk always did kind of bother me. I would tend to think that patriotism would entail love of country, but when it becomes tied to and required to also hate certain other countries, then patriotism becomes complicated and problematic.
So, to answer your original question about "Why is it okay for USA...?", the only rational and honest answer is that it's most definitely not okay.
But to many Americans, it doesn't matter, since many are persuaded to believe that we're on some holy crusade so that the forces of good can prevail over the forces of evil. They're involved in a much higher cause than any of these foolish, naive liberals and other peaceniks can possibly fathom or understand.
