It's important to be able to limit the login for only those from an organization.
This eliminates the need of manually disabling accounts of people who doesn't work at the company anymore. If their account is removed from the github org, they can't login to jenkins anymore.
I agree the current default is a major security issue. It's reasonable to expect that logins are restricted to an organization by default. Since anyone can sign up for a free Github account, the current default is essentially to allow public access to Jenkins-- NOT SECURE.