Okay, so I'm a bit science and religion obsessed academically, but I think the discussion here could be fruitful. This really goes back to the class discussion about the "scientifically verifiable theory of morals" (p. 118). I think Riceguy20 is totally right to highlight the quote "Man has no moral instinct" (p. 117) as really important -- through the instrument of Mr. Dubois, Heinlein constructs the ultimate 'science trumps religion' universe. My interest derives also from Asimov's "Foundation", in which a branch of mathematics called "psychohistory" develops that governs nearly every part of life (to certain scale/degree).
In our present-day world, writers like Edward Wilson might suggest a
biological basis for morality, but even so we're still one step shy of explaining all subjects with symbolic logic. Repeatedly, such logic is the ultimate appeal in Starship Troopers. Assignments like "Bring to class a written proof, in symbolic logic" (p. 179) often follow many enormously important discussions in History and Moral Philosophy. This type of proof (or even just the suggestion that such a proof exists) seems to apparently putting the matter to rest; regardless of its controversial, political, and subjective nature.
Does this strike anyone else as a perversion of the humanities? Not everything can be an "exact science" (p. 182) with one right answer. I cherish needing different models (yeah, there's some truth in looking at morals as deriving purely from nature and nurture; but I also find much meaning from considering some transcendental role), so it frustrates me that Starship Troopers describes a 'one right, proovable, answer' world. Interestingly, your answers in such a universe are still going to depend on your initial assumptions... you can't prove everything. The biggest assumption made, that science works and teaches them Truth about the real world, can be neither proved or disproved, only debated and assumed.
Why might science fiction like ST or Foundation disregard any need for non-scientific truth? Yes, the intended audience might be more familiar with science as authority, but I don't think that's all there is going on here.