It's legal because of option #2.
http://c28157.r57.cf1.rackcdn.com/...pstakes-rev-2nov.pdf 5. How to Enter: NO PURCHASE IS REQUIRED TO ENTER OR WIN THE SWEEPSTAKES. Individuals may enter the Sweepstakes in one of two ways:
(1) By registering, by December 31, 2011 via Active.com, to participate in the 2012 Ironman® St. George triathlon. The following personal information of the entrant will be required to be disclosed in order to register for the Sweepstakes by this method: full legal name, email address, mobile telephone number, date of birth, and the state in which you live. The prize winners will also be required to provide his or her mailing address and other information needed or useful in order to facilitate sending and receipt of a prize.) During registration, entrants may be offered the opportunity to opt-in to special offers. These offers are optional, and neither acceptance nor non-acceptance by entrants will affect the chances of winning the Sweepstakes.
(2) By mailing a 3” x 5” post card, with a handwritten request for an entry in the Sweepstakes and a statement of the entrant’s full legal name, email address, mobile telephone number, date of birth, and the state in which the entrant lives. Such post cards should be mailed to “Ironman® St. George Sweepstakes Entry, 2701 North Rocky Point Drive, Suite 1250, Tampa, Florida 33607” and must be post-marked no later than December 31, 2011 (and in all events must be received by Sponsor by January 7, 2012) in order to be eligible. Each such post card received will entitle the entrant, if otherwise eligible under these Official Rules, to one (1) entry.