Of course they're f***ing liable!
Someone has to filter this crap out. The ad networks (and Google are by no means the worst offender here) can't say that it's down to the ad providers. In the cases we're talking about here, these are the very scrotes punting the crap in the first place. Any solution that revolves around saying after the fact that you're very naughty boys and that we won't be accepting your nasty ad any more is tantamount to a "fill yer boots" statement to the bastards.
Let's face it. It's a *serious* risk when you can get screwed by following a link displayed on an allegedly trustworthy site and it's the ad servers that provide the sewer* here.
Google. Get of your feckin arses and direct some of that innovation to finding a permanent, workable solution to this one. Then share it with the other ad networks**. You'd be doing the web far more of a favour than fannying around with yet another sodding browser that we need slightly less than a hole in the head.
Why would you want to? Because the other solution is a wide deployment of AdBlock, IEPro and their ilk and if you don't act, everyone else will.
*Sewer. A fat pipe for sewage.
**NB: Yes, I know, Not Doing Evil does not oblige you to Do Good, but give it a try anyway.