I am writing this because I just read like five reviews that completely missed the point of the film. It's a parody of the book; it's talking about the fascism the book embraces. That's why it isn't a direct adaptation of the book. The Navy uniforms are deliberately modeled on the Luftwaffe's WW2 uniforms, and the gray of the infantry is also meant to invoke the Wehrmacht. The "service guarantees citizenship" slogan is all about the fascist idea of the soldier as being socially superior to others. It's why the freakin mormons run the heck away.
The humans annihilate the bugs, without seeking to ever really understand them, except as a means to be better at killing them. Which should make one ask, if there is such a thing as a brain bug, can they not be reasoned with? The answer, of course, is maybe, but our fascist protagonists don't care either way because violent annihilation is what they think the whole point of living is.
It's a film that makes out a deep critique of a certain American idea of fascism and civic virtue. It's why it's uncomfortable, and it's why it is the way it is. And it is perfect.