Why would Tumblr be singled out when Bing gets away with it? Bing goes so unnoticed that their image search is practically a porn engine (if you turn off safe search), and I've never heard an outcry about that.
Every female celebrity or politician has "nude" or "naked" somewhere in the suggested searches. To illustrate my point, do a Bing image search for Bill Clinton. Next, try it for Hillary. Not quite as bad, but try image searches for Barack and Michelle Obama. The top three suggestions for Michelle are Ass, Booty, and Butt. They may argue that they're just reflecting what users search for, but I have trouble believing that a lot of people are out there looking for nude photos of Nancy Pelosi.