It's instructive to think of the cases where Google can return a search result, even though the searched word doesn't appear on the page. Most often, this occurs because another site includes an outlink to the page, with the searched word. That is, they're 'copying' a publicly-available source that indicates that word is associated with that page.
I see this Microsoft tactic as similar. They're considering search terms that resulted in a visit to the page from other search engines as being important indicators of the page content. If they have that URL-to-URL-trail data legally, and the signal works well, and they are not singling out Google's URLs as the only source of such a signal, I'm not sure what the problem is.
Google didn't get where they are by throwing out legally-collected useful data, and Bing won't catch up to a leader who has clicktrail sensors everywhere, via analytics/toolbar/ads/mobile/etc., by throwing away legally-collected useful data.
I see this Microsoft tactic as similar. They're considering search terms that resulted in a visit to the page from other search engines as being important indicators of the page content. If they have that URL-to-URL-trail data legally, and the signal works well, and they are not singling out Google's URLs as the only source of such a signal, I'm not sure what the problem is.
Google didn't get where they are by throwing out legally-collected useful data, and Bing won't catch up to a leader who has clicktrail sensors everywhere, via analytics/toolbar/ads/mobile/etc., by throwing away legally-collected useful data.