Let's add a third: security. You can't let a 3rd party advertiser send arbitrary web content to your customers, and then just wash your hands of it and say "It wasn't us, it was the ad network" when it eventually gets used to deliver malware to your customers.

It's like if you hired a housecleaning service and they told you "Whoops, looks like somebody took a dump on your living room floor. Not our fault though. It was a subcontractor."