Do you understand the purposes and mechanics of sage?
Sage is meant to be added to a post the poster deems not worthy of attention and not worthy of bumping the thread to the top of the index.
Naturally, the poster doesn't necessarily want himself to be associated with such a post. This is why sage goes into the email field, so that one cannot accidentally link their email to a not worthy post.
This disassociation is also why sage turns (or used to turn) your ID into a generic one, like Heaven or a bunch of zeroes.
Then, why is sage visible? Because the mailto link for email addresses is. Sage means "mail to no-one, not to me, I didn't post this".
Also, on some boards of current 4chan, sage no longer preserves bumps. Saging still contributes to the bump limit, without bumping. 4chan is taking the feature closer to being a downvote and I wouldn't follow its example.