According to an official Microsoft Support document, "customers can see the item in the Web Marketplace but cannot purchase it unless they have the special pre-paid token ... The content is available only to users who have a special 25-character pre-paid token that enables them to download the item."
It's likely that the full version of Halo Reach is being offered to reviewers via digital distribution; however, don't expect Halo: Reach to be available via the Games on Demand service. Microsoft had previously enabled early digital distribution of Crackdown 2 for reviewers -- and Crackdown 2 is still unavailable to the public via Xbox Live.
Update: A Microsoft spokesperson has offered this statement on the matter: "We plan to provide a limited number of copies of Halo: Reach using the Games on Demand technology for media review purposes only. While we are thrilled to be able to take advantage of the technology to provide media with a more accessible and convenient review experience, there are no plans to launch Halo: Reach to the public through Games on Demand."