I don't think Microsoft will be using that argument anytime in the near future, the public backlash would be huge and if they were knowingly indexing it they would still be subject to legal liability - heck even if they were unknowingly indexing it they could be liable for negligence if due care wasn't taken to prevent these kinds of images from being circulated.

I think Microsoft would argue that there is no real way to prevent these images from being indexed considering that most indexing is automated and it's hard to train their systems to recognize offending images and their human employees can't possibly look through all the images to prevent child porn from being posted.