Earlier quoted context omitted.
Microsoft do NOT steal Google's search results. They use clickstream data from Bing toolbar users to improve their search results. What happened is that Google engineers engineered a use case, in which signal from those clickstream appear like a stealing of search results. Or in other words, Microsoft uses Google users behaviour in their ranking algorithm. But that's not unethical, and Google is doing the same thing.
But the effect is the same as intentional scraping and outright stealing: search results that could have only come from Google are appearing as Bing results. Bing needs to blacklist Google from its clickstream. Simple.
That's one effect. While it's vivid, it might be a tiny side-effect only notable in contrived cases.
Overall, this kind of URL-after-URL signal, extracted from every participating user, and every trail through both search sites and non-search sites, might be discovering valuable terms-in-preceding-URL-to-later-visited-URL associations. These associations might result in many search improvements, other than the one-for-one result porting Google's experiment has found. We don't really know the relative magnitude of porting-results versus other-benefits, yet I think that's important to the analysis.
If a useful automated or user-driven process generates a little indirect infringement around the edges, is that enough to demand the process be stopped entirely? Note, that's not the standard Google wants applied to user uploads to YouTube, or excerpting of news and websites onto Google services. Google says: "defend yourself, by adding opt-outs (robots.txt) or sending takedown notices, and we'll undo the incidental infringements eventually".