Pages whose ancestors are not all "published" cannot be used as parents for other pages.

[caesarsgrunt]

Pages with trashed parents cannot be used as parents for other pages (they do not appear on the list).

Correction : This does not only apply to trashed posts.

For example, changing a post's status to "pending", "draft", or "trash" will make all that post's descendants unavailable for use as parents for new posts. Also, although existing posts with them as parents will continue to work perfectly, the parent will be removed when these posts are next edited.

It's not clear to me what the problem is.

Pages with non-published parents can't or shouldn't be allowed as parents?

Create and publish two pages; Parent and Child. Set the parent of the latter to be the former. Now delete/un-publish Parent.

In the database, Child is still a child of Parent.
In the front end, on the live site, Child will now appear as a top-level page.

If you were to re-publish Parent, the taxonomy would be the same as before you un-published it.

Now there are two problems.

1. Create a new page. Call it Grandchild. Now, although Child is still visible in the front-end and back-end, you cannot set it to be the parent of Grandchild.

2. Edit Child. Save it. Now, the database will be changed so that Child is a root page. If you were now to re-publish Parent, Child would no longer be a child of it.

Let me know if you want any more details.

Pretty clear now, thanks.