Due to CAN-SPAM laws we cannot allow for non-logged in users to opt back in.  In the above scenario, if the donor is not logged in, but rather just enters the same name and email address for the existing constituent record it will put the transaction on that record but not change the opt-in status.  In order for the constituent to opt themselves back in, they would need to log in, then donate.  They could also access their profile to make the change there.  This safeguard is in place to prevent somebody who has the constituent's name and email from opting them back in without the constituent's permission.