"Registered Rabbit cardholders not required to present passport ...". That's what they show at the ticket booth. So as they already have you in their system and can watch your travels on Skytrain, no need to ask for registration again.
More surprising is, that this is "In compliance with the Anti-Money Laundering Act, B.E. 2542"! Money laundering with BTS Rabbit card? And the act is over 20 years old - and only since this year they ask for ID or Passport. TiT!