From their homepage: "Similarly, at our prototype's present levels of scalability we prefer that you not post Arbital links to Hacker News or Reddit front-pages."
Arbital co-founder here: once we upgraded our DB, we were able to handle the HN traffic pretty well, so we are going to remove the "don't share" warning.
Thanks for posting it though!
As we all know, "nobody reads the directions". I can't decide whether to upvote this or flag it.