You can't just send a user directly to that page. They need to have already authenticated with LinkedIn for that page to work, as we pull information about the user from LinkedIn to prepopulate some of the fields and information on that page. If they get there without already authenticating, there's no method for them to authenticate through LinkedIn, which would cause other problems.
For standard Joomla registrations, you should use Joomla's built in process.
Hope that helps explain,
Alex