They don't have to switch to bing
When they log onto Windows for the first time they have IE and Bing.
They download Chrome. On purpose.
They switch IE to use Google. On purpose.
Or even download Firefox, and switch to Google.
Or for the most limited people, they will type google.com every time. I know people who do that.
They do it because they LIKE google, and Bing/Yahoo sucks.
Bing has just as much advertizing stuck at the top, and even more crap links on the first page like cnet "reviews" or ebay links.