re: Alternative option
So if someone deletes a tweet, you're expecting Twitter to go back through all of its backups to delete the tweet as well? I don't normally even do that for files I delete in backed up folders when the back up is attached or live on the server, let alone going through my archives to delete every single instance of the file.
Then there's also the issue of Google cache and a number of other caching sites (Internet archive anyone?). Twitter should act as the user expects it to, so change the 'delete' to 'Make slightly harder to find'. Let's face it, once you post something on the internet be it via Twitter, Facebook or your own website, it's there to stay and there's nothing you can do about it any more in the same way you can't unsend an e-mail or stop a snail mail letter from going out once you post it.