I'm actually totally stumped by the whole thing. OCR doesn't even make sense, because OCR is terrible at handwriting generally. With forms they usually require you to write block letters and numbers inside of a kind of separate grid for each field. And maybe fill in some bubbles too. Anything anywhere on the page outside of the form fields is ignored.
I'd find it far more plausible that they print all letters, those including forms and not, on the same template, and that returned forms get some kind of bar code or status stamped on the bottom upon being received, so they need to keep it empty for that. Kind of like how US envelopes get a little bar code printed on them by post office sorting. I have no earthly idea whether that's closer to the real reason though.