It is because the book is very controversial. Heinlein when he was alive had probably the wierdest rollercoaster of political ideologies ever, from communism to fascism and almost everything in between. So nobody is entirely sure this blatantly fascist book is satire or not (Poe's law should be named Heinlein's law), though most would rather said it is serious. But Verhoeven clearly either interpreted the book as satire or wanted to reinterpret it as satire. Just look at the recruitment scene (and many others), no chance it isn't satire - even though we now live in the onion world where liberals are rapidly reaching that level of caricature, the movie was 25 years ago and back then it was absolutely obvious..