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.
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".
The google engineers intentionally sent this click data to Bing, so is Bing really stealing? It's odd to act surprised when Bing uses the data that was intentionally sent to it. Bing could specifically ignore Google search results pages when it is tracking clicks, but is that legitimate? Google scrapes everything, why shouldn't Bing?
In fact, the data that it takes from Google engineers for carefully engineered corner-case searches is the exception.
edit: I should have clarified. I know that the Bing crawler likely respects robots.txt, but if they are using clickstream info to build their index, it seems right that they should respect robots.txt there as well, no?