That's not exactly the behaviour.
What they do is they insert an additional block that vaguely resembles Google -all white, input box centred under a generic image which could maybe remind you of some Google doodle- right below the header and above the search results. [0]
That input box is empty, but the input box in the header is still populated with your query.
That input box is also generally similar to Google's but then again it's the exact style they already use in their -Bing's- normal search box, just with an all white background.
Something they do is they scroll right to the inserted block so you don't see the Bing header. But, sincerely, saying that this impersonates Google or that it misleads people into thinking they might be on Google seems fairly dubious.