New Pro Site User being added to primary site and their new site

I've been testing setting up new sites with Pro Sites plugin, and what I realize is that when I trigger a new site through the network admin, the new user of the new site being created is added to both the primary "" site and their own new "" They also receive an email from both sites as a new, registered user.

Why is that? How do I restrict the new site owner to only be added to their single site instance?

I don't want them to be able to toggle between their new site and, because there is no reason for them to do so - is the "sales" site to get them to sign up. They shouldn't be registered for that site. They should only be registered for the site either they pay for and create through the payment system or I trigger from the Dashboard Admin.