I'd venture to say this fell under a "Member Exclusive Offer" (usually included in your paper statement), although I have the October 2004 MEO list (last one I've received) and this particular offer doesn't appear.

Edited to add:
Just found this listed on the Marriott Rewards Specials and Packages page for CT:

The Mystic Marriott Hotel & Spa offers a terrific weekend rate of $149 per night (Fridays and Sundays) PLUS you'll receive 500 bonus points per stay now through January 31, 2005.