I bet it was.
Now that I have read my own post, I don't think I even answered the question.
My next effort: when в дневник, в журнал are being used, we want to convey the meaning of putting words into a container.
And the opposite case: normal writing is not to store words but rather to share, so a letter is not a container as such. Therefore, писать в письме, not писать в письмо.