Are you sure? From https://blogs.bing.com/search/2011/02/02/setting-the-record-... (context: Google did a sting operation by faking search results for made up terms, Bing ended up showing the same results):

"We do look at anonymous click stream data as one of more than a thousand
inputs into our ranking algorithm. We learn from our customers as they traverse
the web, a common practice in helping to improve a wide array of online
services. We have been clear about this for a couple of years (see Directions on
Microsoft report, June 15, 2009)."

And from here: http://searchengineland.com/google-bing-is-cheating-copying-...

> Indeed, the statement that Stefan Weitz, director of Microsoft’s Bing search engine, emailed me yesterday as I worked on this article seems to confirm the allegation:
As you might imagine, we use multiple signals and approaches when we think about ranking...
Opt-in programs like the [Bing] toolbar help us with clickstream data, one of many input signals we and other search engines use to help rank sites.

Also (same link):
> Microsoft does disclose that Suggested Sites collects information about sites you visit. From the privacy policy:
When Suggested Sites is turned on, the addresses of websites you visit are sent to Microsoft, together with standard computer information.
To help protect your privacy, the information is encrypted when sent to Microsoft. Information associated with the web address, such as search terms or data you entered in forms might be included."

And (refering to Bing toolbar):
> The install page highlights some of what will be collected and how it will be used:
improve your online experience with personalized content by allowing us to collect additional information about your system configuration, the searches you do, websites you visit, and how you use our software. We will also use this information to help improve our products and services."

From the same article (at the time, in 2011) Google deny using their services to index search results, FWIW. But AFAIK Microsoft never made such a clear denial - do you have a reason to be so confident in your claim?

There's also this followup: http://searchengineland.com/bing-why-googles-wrong-in-its-ac...

> Bing says it does NOT do this. It says there is no Google specific search signal that it being used, no list of all the popular pages as selected just by Google users. Instead, it has a “search signal” based on searching activity observed across a range of sites.
For example, if you did a search on Amazon, Bing might detect that. A search on eBay might get spotted. A search on Yahoo, that also might get extracted. Any number of searches might be identified. Bing would associate the next page you went to after doing those searches as being a possible “answer” to those searches.

> “We aggregate the information,” Shum said. “The entire clickstream gets weighted along with different signals,” he explained. “For head queries, we have more signals. For tail queries, we have less. For the Google ‘synthetic’ queries [done for the Google sting operation], we have nothing.”

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So Bing seem to, as a ranking signal, capture URLs that look like searches, and then the next page - which is pretty clever. If that next page is not already indexed (or alternatively perhaps if they have no other ranking signals for the search they spotted) they seem to index that site as part of ranking that signal. And that was 2011, who knows how much cleverer Google and Bing have got since then.