If you prepare a ligit-looking web page where you instruct people to download and run malware, we'd better learn more on security and caution before blindly follow those directions.
Why should it be Google's (or Bing's) duty to filter those out?
Google intentionally disguises ads as search results, and even lets advertisers present a fake URL. When the system's purpose is to profit from tricking inattentive users, I think they should take on some liability for the outcome of what they're tricking people into doing.
Not to say that better teaching security isn't also a good idea.
The answers are in the question.