I don't understand why people wouldn't take it at face value without any further context offered to them beforehand. It's a campy action film that's a lot of fun but with just the faintest dusting of authoritarianism that could be easily disregarded as just part of the ambience of the filmmaker's decisions. It ends on a hopeful scene implying better understanding of the Bugs in order to reach victory.
You've got to read the book to get it, but even then that doesn't really shift the film out of being a Big Action Shooter that's fun to watch.
I think what made it an obvious parody was the over the top PSA/commercials they broadcasted, and the fact that citizens are treated as a privileged class. I think at one point someone mentions that they have to become a citizen in order to be legally allowed to have children and that's why they joined the army. It's so far abstracted from our own reality, that I even picked up on the fact that it was a parody watching it for the first time at 12 years old.
TBF but a parody of what? Sure it has a satirical look and exaggerates aspects of modern society, however we are trying to balance that against the book.
We could even say that the movie is taking shots at Heinlein’s own personal beliefs because arguably Heinlein’s personal politics paint him as a rabid anti-communist, pro-nuclear and -projection of force.
Personally I viewed the commercials and over the top stratified society based using a trope of a somewhat Roman or Spartan militaristic society, but it’s been a long, long time since I read the book to remember enough to actually compare Heinlein’s politics to the book and then the film to that book and modern society.