If you look up some of these letters you'll see they have the quasi-official-looking things you'd otherwise see on scam letters, like a stamp that says "Enforce

If you look up some of these letters you'll see they have the quasi-official-looking things you'd otherwise see on scam letters, like a stamp that says "Enforcement Visit Approved" with a signature on it. I think "do not write below this line" is just another one of those things, it makes the letter seem like its part of Official Serious Bureaucracy.