Twitter does not have any method to request the user's email address. There's no way around this, so we must present the user our min-registration form to ask for at least their email address. In our settings area, you can disable asking for name, password and other data so they registration process could be as small as just an email field.
I hope that helps explain. We'd love it if Twitter would return the email address as well for a more streamlined process like most of the other networks.
Thanks,
Alex