Non-free playgrounds have additional protection in place to prevent cheating and unfair play. On non-free playgrounds, you can only have one simultaneous game session per account. This means that if you are currently playing on a non-free playground, and your friend joins the same non-free playground using your account, the old session (which would be your session) will be disconnected. The same applies if you try to join a playground multiple times in different browser tabs.