Most actions are reversible: delete/undelete, edit/rollback, close/open, protect/unprotect, vote, etc. But tag synonymization isn't. There are cases where the community thought an idea was fine just to discover years later that it was not.
This can only be addressed by a moderator, which can take time and effort from the team. Since moderators are supposed to do as little as possible, community shouldn't bother them if they can clean up their own mess.
If a tag synonym was created and later considered a bad idea, this is obviously a more complicated case. Synonyms can be tricky, and I personally expect any non-obvious synonym to be vetted by a meta discussion.
The actual act of creating the synonym is not much work, it's just a few clicks. Crafting a convincing argument for or against the synonym is more work, but that is something the community can do, and in most cases already does. And leaving the ability to undo synonyms with moderators makes sure that a meta discussion takes place.
There's another reason that undoing a synonym should be discussed publicly and not just be a vote somewhere. You're going to have to manually undo all the damage the bad synonym did, and that needs a meta post in any case.
