When double opt-in is enabled and a person confirms their subscription via the confirmation email, the 'Do not allow bulk emails' field on the Contact record should update to 'Allow'.
Currently, if you set that field value to 'Do not allow' by default then the Contact will be added to the subscription list but not receive the email.
If you set the field value to 'Allow' by default then they may get used in a segment before they have confirmed their subscription.