Maybe it is because it is a custom add-on and they can't implement too much fine control on it, everyone with level 4 access can create edit and delete canned replies (no matter if they are from someone else) so they have to be sure people with access are going to behave and make good use of the tool. I think that is why they only grant it to people that has already proven to be trustful.
I'm speaking for myself of course, I'm not moderator or administrator of the forum.