Given that IE has always pushed Microsoft's search engine (previously Live Search, and previously to that MSN search), the logic here appears to be flawed. If anything, Microsoft has been giving users more options to switch their search engine in IE.
On the other hand, there is this data: http://www.techspot.com/news/43155-users-with-newer-ie-versi...
If you're too lazy to read the article, it suggests that newer versions of IE have more Bing usage, despite the fact that they let you switch your default search engine.
It seems that there are more IE users that previously did not use a default search engine and when they upgrade their browser, they end up using Bing because Microsoft sets it as the default.