I'm mostly indifferent and it is very simple to change. It seemed desirable to have the main topic of this page display as something that isn't invisible though. And NULL was unappealing since it naturally appears in data. Since COPY uses \N I figured it was a decent choice. <null> or <NULL> came to mind too, but there were long compared to 1, 2, and 4 that appear along side it.
You decided to show \N for Nulls. I think that the user who has questions about nulls is usually a newbie, so he also doesn't know the COPY format for nulls. And even GUIs, which are often used for learning, display a word for nulls, be it NULL, <null>, [null] or something similar.
