Basically, users were deleting images they uploaded because they got upset with mod decisions, or because they didn't want them to show up in google searches for their name, or other stupid reasons.
But that meant that other people couldn't re-upload the posts, or a mod had to purge the DB entry entirely causing loss of all assocated tags, notes, favorites, scores, history, etc.
So it's something that should be prohibited, except in unusual circumstances in which one should contact a mod/janitor to delete the post. Although it'd be fine to allow self-deletion for a short while in case of accidental uploads, like for a few hours, and prohibit it afterwards.