That looks to be a bug-ish thing. The hiding of the register button only works for the login form itself. The modal is it's own little thing, and, frankly, we just always assumed the register button should show in that mode. I can see adding another option for hiding the register button in modal as well as hiding register in the login view.
To fix for now, edit the /modules/mod_sclogin/tmpl/login.php file. At line 14, you'll see the following lengthy line:
echo '<div class="sclogin-modal-links"><a class="btn btn-primary" href="#login-modal" role="button" data-toggle="modal">Login</a><a class="btn" href="' . JRoute::_('index.php?option=com_users&view=registration', false) . '">Register</a></div>';
Update that to the below, which simply removes the Register button:
echo '<div class="sclogin-modal-links"><a class="btn btn-primary" href="#login-modal" role="button" data-toggle="modal">Login</a></div>';
I hope that helps, but please let us know how it goes as well as any other feedback you may have.
Finally, once you've tested our free SCLogin module and are happy with how it's going for you, please consider leaving a rating and review on the Joomla Extension Directory for it. It's certainly not required, but very appreciated, and you'd be the first to review it!
extensions.joomla.org/extensions/access-...authentication/24054
Thanks,
Alex