Why do Contacts remain active when linked Company has been deleted?

I just had a look at the list of Contacts stored under my development account. There are many (more than a dozen) Contacts listed, with active = 1 and is_primary_contact = 1.

These contacts previously belonged to Clients which were deleted more than a week ago.

I expected the deletion of the Company to cascade delete the related Contact.

Is this as-designed and as-documented?

It would be nice to be able to see some kind of audit trail on the data save/edit/delete sequences.