Activity Contact (& Activity?) records not being deleted on contact delete

This was just reported on IRC & I checked a live DB & found it too - running this query

SELECT count FROM civicrm_activity_contact ac
LEFT JOIN civicrm_contact c
ON c.id = ac.contact_id
WHERE c.id IS NULL

Returned many thousand records.

It appears there is no Foreign key to cause deletes & presumably no alternate mechanism in place