If you are a registered user logged onto the SE network, say to UX.SE, you have a user account with reputation and badges, etc. When you go to check out another SE site (cooking.SE for instance), at this moment, you'll be treated as if you were a new user.
Do you think that a registered user should be treated as a new user when visiting another site?
Edit I'm not an advocate for automatic user account creation. I think a 'visitor' role would be a good idea. When a registered user goes to another site, he/she will have a username and a 'create account' link or something. No reputation, no badges, but yes a username and a single click to create the account. The visitor can remain 'anonymous' to the new site, but the site does know he/she exists as a registered user somewhere else.
Disclaimer I've posted a very clumsy question about that on MSO.