Never underestimate the ability of a human being to rationalize. If I was in the Microsoft camp, I'm sure I would also be grasping at straws to explain why it's totally fine for Bing to use Google's search results. It's human nature to rationalize. The bottom line is that Bing's index contains associations that it could never have figured out if Google hadn't figured them out first. How many there are, we cannot know…
Yes, Bing toolbar just rely on people clicking links. But without Google doing all the hardwork here, users will never know about such link, that's why they went to Google. And there will not be any of such link to feed to Bing's system.