Oh, dear, another soul who's confused what the book actually says with what they want to agree with, namely, the politics.
What happens to the hero is far more important than what gets said, even if all the little lectures seem to constitute the bulk of the book. After Rico takes the aptitude test, he indicates his preferences for which branch he's to go into. He put MI at the bottom of the list, even after logistics, which he doesn't even know what it is. When the interviewer finds that Johnny wouldn't sneak in a dog, he announces that Rico's only choice is MI. Since the whole point of the what little story there is, is that Rico is pretty much a complete horse's ass until the Army makes a man out of him, it is plain that MI is the bottom, for the no-talent dogfaces, the grunts.
(Or seemingly, at least, until Heinlein's finishes glorifying said grunts, the poor downtrodden infantry. You should imagine a music video at the end, with beautiful shots of glorious heroes, while the sentimental Ballad of Rodger Young plays. Think Alvin York as played by Gary Cooper, maybe. This glorification of the military is indeed militaristic and in that respect everyone connected with the movie is correct.)
If Rico wasn't good enough for the MI to start with, then how is he good enough when he quits MI? If he was just being tried out for MI, then the 90% attrition makes perfect sense, the real point being military indoctrination. Speaking of which by the way, telling children, even snotty adolescents who have crazy ideas about knowing everything to present logically organized and soundly sourced argument orally,
ex tempore, no less, is
not a simple pedagogical technique. It teaches the majority not to question the official views, lest they be put on the spot. A handful will enjoy the spotlight, but in Heinlein's world, the word of mouth effect from the HMP professors is going to haunt them for years. Think of it as political provocation by the authorities as an official part of the school days!
You do realize that if the Federal Service is really so small, so parasitic, you're saying that the electorate must be tiny? Poul Anderson, bless his heart, was a political nincompoop of the first water himself, see People of the Wind for one, and the agreement that Switzerland was in any way comparable is insane.
The book's alleged world is internally contradictory, senseless when compared to any real life, a political swindle initially aimed at children. The poster above who thought Heinlein was only motivated to fuck with children's minds because he wanted a little beer money may think that justification, but I don't. Heinlein put in little justifications for his crazy world so imperceptive readers (aka children) would understand. But there really does come a time to exercise mature judgment.
The world Heinlein imagines can't work. What's left is a glorification of the military, and ferocious insistence that race enemies must be destroyed. You may argue that it isn't fascism without a dictator, but you'd still have to concede Heinlein's Federation is more like the Romulan Empire than the USA.
PS
As to not understanding the oath, and that being fascist in spirit. Every democracy on Earth today has requirements for voting concerning age, birth and civic status. Are those things fascist as well? Why should a Down's syndrome person be able to vote when a perfectly capable 14 year old cannot?
Civic status is a cute little evasion. Racist laws that aimed at African Americans end up disenfranchising them. Technically, like literacy tests, it has nothing to do with racism, but the practice is racist and, therefore, yes, is fascist in spirit. Age has to do with independence. I have no idea where these perfectly capable fourteen year olds are lurking (behind the scenes in Hollywood? That would explain a lot, no?) but the real point is that the franchise is automatically granted. If you can convince people that fourteen year olds are responsible then they too should be enfranchised, automatically. That means the ability to sign contracts as well, after all. My experience is that adults who are truly not capable of managing their own affairs have little interest in politics and therefore would generally not trouble to exercise the franchise. And, considering their relatively small numbers, and their natural diversity, they certainly wouldn't be some sort of special voting bloc deranging the democratic process. I can't help but suspect that behind the lust for a restricted franchise is the desire to limit personhood to people of the right sort. Which, again is undemocratic and yes, trends toward fascistic.
Hendricks being unsuitably insubordinate for a battlefield is one thing. The conclusion that he is therefore unsuitably insubordinate for the franchise doesn't follow at all, unless one aspect of the system is to ensure that the electorate has been suitably indoctrinated to follow discipline. And, yes, again, that trends to the fascistic side.