When people login using facebook or twitter account, after that why they cannot login via login form (using username/email : password). Is that mean all the members from facebook or twitter need to click the facebook/twitter banner everytime they want to login? I'm confius.
Yes, it is. And that's the way they work. In most cases, users register with the same email as they use on facebook/gplus/twitter. That's he can't register as a normal user with the same email.