It already IS an open SSID - the portal you sign in on only controls whether their system lets your connection go through to the public internet or not. Since it's an open network (IE, not secured with a password/WPA2 key), everyone can see everyone else's data packets. You're relying on either your own VPN or an encrypted connection to the website you're accessing for security.

The only way they could defeat that would be to give everyone a unique username and password to log in to the Wi-Fi with, which would not be practical.