deleting civicrm_contribution rec doesn't delete associated civicrm_line_item

Create a price set. Create an Event page that uses the price set. Create a registration for the event that includes line items from the price set. Review db to determine id of new civicrm_contribution record and find associated civicrm_line_item records. Delete contribution. Check db: civicrm_line_item records are still there though the civicrm_contribution record is not.