This is only my observation / hunch, but when SSO is enabled AND you supply a username / forum name in the auth process, I'm pretty sure the presence of a forum name skips Lithium's secondary registration page which normally appears for first time visitors. This page requests a forum name and includes the T&Cs agreement.
I think you're right cblown. We felt the same too and hence tried to create user via API without providing "username" - only ssoId and email - so that the absence of username/ forum name would invoke the registration page containing forum name and T&C. But the API returned the following response "<message>user.login: Specify a username.</message>". Although as per API documentation, the forum name is optional.
