When using SSO, users have to have the correct sign in link for the main/sub account there user account has been created in.
We have a main account and multiple sub accounts all using SSO, when we create a user in a sub account they have to use the correct URL otherwise they are denied access or presented with a login box. As we don't share the password with them they are then stuck.
Also having an SSO link button on the login page means there is just one URL for everyone to remember/save, this also helps when using the "share tile" functionality as if you click the View in AlertSite link in the tile and are not already signed in you are presented with the login box and again stuck if you don't have your password.