Hi Kevin,
Its not a bug. The only thing that differentiates a root level page is that it has a parentid of -1 and parentguid of guid.empty.
If a parent page is deleted its children must be de-parented by changing their parent id to -1 and parent guid to guid.empty so naturally they fall to the root. Leaving them attached to ids that no longer exist is not an option so the only other choice would be to delete child pages and that seems heavy handed and could lead to accidental deletion of pages.
Best,
Joe