I got a feeling it sounded rude to the top post, despite me not even having an iota of intention to think that way.
It's not so much that it is rude in the way of typing in all-caps, it's more that it leads to endlessly long chains of quoted posts below the relevant content, particularly in mailing lists that have a digest which delivers a full day's posts all in a single email to users who select that option - where many of those posts will each quote all of the previous posts, even when they immediately precede each other in the digest. It could probably be argued that the prohibition was more relevant in the days before 99% of users read their email via clients which hide quoted content unless it is explicitly expanded. Arguably, smarter digesting software which strips out unnecessary quotations of earlier posts from beneath top-posted content would be a more effective solution now that it is no longer common netiquette (nor all that necessary) to avoid top posting.