That's part of [new user restrictions][1] which for some reason is not documented.

User with up to 10 reputation, including 10 rep will have the link as plain text. [Example][2].  
User with 11 or more reputation will have the link clickable. [Example][3].

The network profile doesn't have reputation of its own.

I think you are correct though, I suggest that unless the user has at least one account with more than 10 reputation, the link will be plain text.

  [1]: http://meta.stackoverflow.com/privileges/new-user
  [2]: http://meta.stackoverflow.com/users/149330/eddiec
  [3]: http://meta.stackoverflow.com/users/136382/rolf