They may just be using terms in the referring URL rather than the referring page. It'd probably yield better data. Most site-specific search pages would have the search term in the URL.
I wouldn't be surprised if they're more interested in data from domain-specific sites like epicurious than generic search sites like Google.
I'd also guess that this won't work once the SEO guys figure out they can feed fake clickstreams to MS.
Referring URL? Oh right, Google intercepts all search result clicks through a redirector, and includes the search term in the intercepting URL. Yeah, the unique words in that URL could figure into describing the content of the final destination page (after redirect), the same way anchor text figures as well.
I wouldn't be surprised if they're more interested in data from domain-specific sites like epicurious than generic search sites like Google.
I'd also guess that this won't work once the SEO guys figure out they can feed fake clickstreams to MS.