It's required, but only if it's visible

I've added a CCK Integer field to the Son content type, and set it to be required.
I've not given users neither the permission to edit nor to view the field.

When the user submits the Add new Son form, she doesn't see that field, and then she doesn't insert any value.
The Son is accepted and no error is raised: into the database, the field is filled with a NULL value.

This means that it's not true that a "required" CCK field is always set (to a not-NULL value) into the database.

Is this a by-design behaviour, or is it an unexpected behaviour?

"It's required, if it's visible" vs "It's always required".

Since I would like to have the second behaviour, must I fill the field via a custom module?

Thanks, Giovanni