The wording could be better, it's meant that they're not liable for hiring kids since they can say that they didn't know, are not responsible for knowing.
I watched this in the cinema with about 5 or 6 friends when it came out. All of them thought it was terrible because it was "praising of militarism" or something. None of them got the satire dripping from every frame. I was dumbfounded.
In later years I found out that this is a phenomenon with not only the movie, but the book too. There's some kind of cognitive disconnect, some inability to see, for what is glaringly obvious to others, the bald fact of the sarcasm and parody of the USA and fascism going on here.
Or normies show up and take over?