This is how the benefit (Platinum would be the same) reads in the Terms and Conditions of Membership:
4. Check-Cashing Privileges. Gold Preferred Guests are eligible for check-cashing privileges of $300 USD or equivalent in local currency per Eligible Stay at Starwood properties, subject to local banking and foreign exchange regulations and continued good standing as a Gold Preferred Guest member. As applicable, checks may be cashed in local currency only.
My guess is there is something in the local banking and foreign exchange laws that prevent it. Could be the same for traveler's checks.
Sincerely,
William R. Sanders
Customer Service Coordinator
Starwood Preferred Services
[email protected]