It's not about the quality of the English as such - it's about the military looking to recruit people who set and maintain their own high standards, who attempt their very best at everything they do, as opposed to only doing their best when they're being assessed, when they think it's important, etc.
An attitude of, "it's only the internet", and "I'll do better when it matters", etc, which is often seen on here from many of the younger generation, reflects on the personal standards of the individual - like it or not.
Then again - maybe I'm a dinosaur. Maybe I'm still thinking about the attitude of the military I joined, as opposed to the military of today?
Standing by for incoming...