PR. To appear to be accepting greater accountability.

Do you honestly think Cook (et. al.) read their own email? I would be surprised if it's not filtered and triaged by at LEAST 3 levels of people before they see it.

It's there, but any rational person must realize that the CEO cannot possibly act faster than immediate supervisors. CEO will, at most, hand the immediate investigation to an underling who will then contact HR. And corporations have had employee manuals long before CEOs started maintaining a public email address.