I may be wrong here, but the malleability of notes has to be approved by relays to ensure no funny business is going on. I'll give you an example. You post a note (url) that says 'like' and 'boost' if you like puppies. Then a day later you change the word 'puppies' to 'government'. This may have a reputational effect on anyone that interacted with your note. Use your imagination on more sinister examples.