We need to first understand what Bing is exactly doing here. Bing Toolbar is studying their customers search habits, not just of those search done is Bing but also Google. The ‘keyword’ log of searches can be useful for Search Engines to study the patterns of searches, quality of results etc. This kind of monitoring is not unique to Bing Toolbar, even Google’s Tool bar does the same.
When a search engine sees an article which is graded high in either Digg, Reddit or Delicious the algorithm ranks the link high in their results. This is true not just for Google but a lot of other search engines as well. From what I understand Bing has a 1000 different factors for ranking a particular page on its site. Out of these thousand signals, one of the factor is what link end users click when they search for a keyword in Google.
What Google has done here is that they tricked Bing’s algorithm in a way that Google’s Page ranking & user clicks on Google’s results to become the ‘determining’ factor for the results in Bing Search,
It might seem little disingenuous for Bing to have Google search engine results to be a function of its algorithm. Which is what Google is trying to call as copying. Bing argument is they are studying user search behavior to better their search results. Technically what Bing did here is no different from what Google does elsewhere.