It is WMF policy to revoke all privileges when staff members depart, even if those privileges were granted prior to the beginning of employment by virtue of volunteer work. Consistent application of this policy helps to protect the privacy of departing staff members: no fault is implied.
Automatically revoking privileges that were given as part of someone's job makes sense. Revoking privileges received as a volunteer seems pretty hostile and not a great way to encourage further volunteer contributions. I don't get the privacy reference at all. Is that about not disclosing the fact that someone might have been fired in a way that makes us not trust them with +2 any longer (and there might be legal restrictions on disclosing even that fact)?