Matt Cutts asked Bing point blank if they are stealing keyword rankings from Google. The question was mostly sidestepped as far as how specific honeypot results ended up in Bing, but he said that, "We learn from customers who opt-in to share the activity they do."

So essentially, Bing watches what their users type into Google searches and apparently look also at what they click on in the results.

Google's engineers working on the honeypot sites and searches were acting as "consenting customers" of Bing.