Are you asking why they can delete the administrator user? If so, it's because you've given them permission to delete users. Vanilla does not have special protection for admins.
I think what @Lincoln is saying is that permission settings are entirely dependent on how the forum is configured. If you give moderators permission to delete users, they'll be able to delete all users.
If you want to keep moderators from deleting administrators, remove their ability to delete users. That's my guess, anyway.
I don't know of a plan to "protect" admin accounts. My advice for now is to not give that ability to anyone you don't fully trust, and keep good backups.
Answers
dangerous permission
If you want to keep moderators from deleting administrators, remove their ability to delete users. That's my guess, anyway.
i think is a firstable duty of moderator