As a casual browser I can only blame the site that "served" me, good and hard. I have no idea what network or subnetwork ultimately served the malware/ad. And

As a casual browser I can only blame the site that "served" me, good and hard. I have no idea what network or subnetwork ultimately served the malware/ad. And since the visited site is the one that came into my "home," it's entirely appropriate to hold the site's feet to the fire. When a site becomes well known for holding itself responsible for what comes out of the site, and doing it effectively, I'll whitelist them.