I think there's a pretty reasonable explanation here, which is that "Do not write below the line" is a genuine instruction, but not for the recipient of the let

I think there's a pretty reasonable explanation here, which is that "Do not write below the line" is a genuine instruction, but not for the recipient of the letter. Post offices may make notes such as "undeliverable" on a piece of mail. The sending company may make changes to their mailers which must be hand-updated on pre-printed cards. In both cases, writing below the line may obscure which ID had its letter rejected by the Post or which IDs have not had updated mailers sent yet. By the time the recipient of the letter receives it, they may write below the line as much as they like, as the instructions have already been followed by those they were intended for. I would not expect first line support to be aware of this.