Enable the "Opt in new users to receive email messages" feature in the site settings and save that change.  New constituents created from signup request transactions received after saving this change will have the "Requests no email" box set to no/false when they are created.

STEPS:
1. Log in to the NetCommunity site as a user with supervisor rights
2. Go to Administration > Sites & settings
3. Click "Main Site" in the left frame
4. Click the "Settings" tab in the right side of the page
5. Scroll down to the section labeled "Email opt-in" and mark the check box labeled "Opt in new users to receive email messages"
6. Scroll back up to the top of this settings page and click "Save" to save this change