The thing is, what if the screenshot is missing? What if it's corrupted? What if the mod is physically incapable of taking a screenshot? The only person who knows when they were banned in that scenario is the banned player. Imagine this: A player is banned on July 18, 2012.. His ban message is "Griefing. Appeal @ Skyblock.net." There's no signature, so the player won't know who banned them. However, if they say that they were banned on 7/18/12, then moderators can sift through and see who was a moderator/tmod on/or before that date. Hope this helps
Nearly all staff have been there since 2012-13. It wouldn't help. And if there was no signature, a mod would take over.