As newt has alluded to, there is a paragraph in the Terms and Conditions regarding the arcade, specifically:
[quote:t36a7gm4]Scirra.com as a service allows anyone to use the 'copy and paste' game code onto their own websites. This allows webmasters to show specific games on their website hosted by Scirra.com. Webmasters who take advantage of these service give total indemnity to Scirra Ltd for any damages, fees (including reasonable costs), consequences arising from using the embed code. People who use the embed code do so at their own risk, and fully understand that these games are provided with ABSOLUTELY NO WARRANTY WHATSOEVER. People who take advantage of this service accept that they will never hold Scirra Ltd accountable for anything resulting from using our services.
So basically, anyone can have access to the games in the arcade.