I understand that, but the option in the UserCP says, "Hide email address from public?" On most message boards I am familiar with, that means even registered public. It wouldn't normally be a problem, but this forum has quite a few registered spammers. Here's just 2 I found in a few minutes:
After doing a little research, I see this is a problem with the forum software and the administrator doesn't have the ability to alter this behavior with the default admin option set. You can, however, install this mod that seems to be very popular: http://custom.simplemachines.org/mods/index.php?mod=371
It turns the email link into a form with captcha and protects all email addresses from being seen, whether or not the member has chosen to hide their address. This isn't the only mod that takes over where SMF left off, but seems to be the most popular.