Let's say I visit a Costco warehouse, and there's a 3rd party vendor there. He offers me a box of pans. I take those pans, the box breaks open and a 20 lbs pan falls on my foot breaking it.
Who is responsible? Costco? Or the vendor? Who do I have an implicit contract with when entering a Costco warehouse?
Same with Google. If the ad downloads malware, we should hold Google responsible.