My guess is that they scan the letters that are returned by the post office and hence they don't want anybody writing below the line. I guess that they used to have a problem with squirrels opening up the letters and scribbling below the line. However, they seem to ignore the fact that squirrels can't read.
More likely is that whenever they print OCR'able numbers / barcode/ whatever, they assume that a person is going to return it -- and the special case of 'we only get returned letters when the delivery has failed and nobody has opened the envelope' escaped the testing.