The Mastodon model does not offer much ability to delete. Well-behaved servers will honor delete requests, but the protocol doesn't mandate it. Additionally, a user cannot generate delete requests if they get banned from their server or the server shuts down. Users and server admins can't control whether another server permits archiving of their content. Mastodon and other fediverse software allows following public posts by RSS, and RSS clients might keep them forever.
The only reasonable understanding is that these protocols are for for publishing to the public. It is not possible to reliably retract anything published to thousands of other peoples' computers. We used to try to teach people that the internet is forever, and that's even more true with federated protocols. That doesn't make them a bad idea.