Hi,
OpenID is an authentication service not a registration service, when we get a post result from the open id authentication process we have no way of knowing whether the user came through via the login page or the register page. So if the user does not already exist we need to create a user and assume he came in from the register page. Otherwise it will not work for people who really do come through the register page since we cannot tell them apart.
Hope it helps,
Joe