Complete user profile after social public registration
Via https://dukt.net/craft/social/docs/templating/craft.social#getLoginUrl you may give up a page like { redirect: 'user/finish' } on which you could check if a user agreed to the license agreement and if they have redirect them to the regular page.

Here is an example profile edit page that might be useful https://github.com/dukt/social-demo/blob/master/craft/templates/social/profile.html