You have a valid point that the click-through measures more of what Google puts out rather than what users find useful. But Bing has access not just to the click-through rate but also the rest of the clickstream. IF (and that's an all-caps if) the value of the click is weighted by engagement metrics on the resulting page, the association is more closely related to the value the user finds in the page rather than the results that Google returned. Would you have a problem with Bing doing that? As a user, and not a developer, I rather like it because it's a much more direct metric of whether a link is useful or not.
They are free to do that on their own search results. If they can't figure out how to rank the document in the first place, they shouldn't be getting data about the association from google.
