It's perfectly fine to say "those Americans" , "Those Danes" , "Those Brits", etc.
All populations share a opinion, or so would it seem for most countries.
Most Americans have the same idea of things, possible duo to their obsession of the Media, nevertheless they think the same, so it's fine to generalize them as one and not as different from eachother.
Ofcourse, every country has a few outstanding individuals whom are not like others in their, intelligence, way of thinking, etc.
You can't say "Those 2.4% Americans" , "Those 100 Danes".
If you speak for/against a people, you have to include everyone of them, as ignorant as it might seem.
EDIT: Sorry, completely forgot to answer the question, ofcourse after my interpretation of the answer.
I don't think it's right. to me it seems as the US clearly did it to demonstrate their power.
The Atom bomb has destroyed many generations, for what? a war that only lasted a few years. To me it is a war crime, what the US did, regardless of what Japan did.