I want it so when you delete a message it disappears right away and doesn’t say message deleted by author and I know that snap devs need to review our messages
I think they do this so people know that it's recently deleted, and make it possible for people to flag, if it instantly disappears it's impossible to 
flag the topic by people.
Keeping the post also allows you to undelete a post if you accidentally deleted it.
If that was the sole reason, they can just make it so that only you can see the deleted message note.