register.json does not login the user or return the user's session/token/password
Not sure if you need an answer at this point (or if this is even one), but it looks like you have "Require e-mail verification when a visitor creates an account" turned on?

In your image, in the second window (where you see the message from drupal_set_message()), it says that an email was sent and you will need to follow instructions to gain full message. 

To turn this off, navigate to /admin/config/people/accounts and uncheck Require e-mail verification when a visitor creates an account.

If you look at the user table in the database, the status column should show a 1 for enabled, 0 for disabled (meaning he hasn't clicked on the link in the email). 

Hope it helps!