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There is absolutely a reason. A user queried for a string, then followed a link. Biasing Bing's search results towards the followed links is a signal that improves their search.

Yes, but the actual effect is that they've just copied Google's results, rather than extracted valuable insight from their own users. And I don't see how Bing could have scraped the search terms and results from a Google session unless they are using code specifically tailored for Google's site. Given that, the clickstream excuse rings awfully hollow.

Plagiarism is going to be a judgement call in a developing field like search. Google's judgement is that Bing's scraping of their site, in this case, is fairly underhanded. I would agree.