I guess you haven't tried logging in as a user with less than Administrator rights or tried logging in to someone else's forum. If you did then you would find that while people can examine the code, they cannot edit it.
It's part of Nabble's way of showing newcomers how wonderfully flexible and customisable their forums are.
As an administrator you could edit the NAML code to remove it - but then you lock yourself out of what may turn out to be a necessary facility.
Had you searched the forum fort "edit this page" before making your post you might have found your point made and answered several times before. You might also have found:
http://support.nabble.com/Edit-this-page-only-for-admins-tp7591622.html(Apologies if I sound grumpy! It's late here and I really ought to go to bed!)
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.