I never claimed it was a perfect analogy and that, as it is a grey area, the responsibility (and therefore the potential liability) isn't always clear-cut.
The problem with the counter-scenario you put forward is that the website is not 'offering' the potentially dangerous content (ice-cream) to the visitors; it is offering some wanted content and then more-or-less forcing the visitor to be exposed to some other, unregulated, potentially-dangerous content.
As I said, it's not a perfect analogy and I used it only to explain how determining responsibility is very dependent on the specifics of the situation.
It matters not one whit that the content on the site is free to consume - the site can still be liable. To pull another (imperfect) analogy out, consider a marquee setup in some public place - a mall or thoroughfare perhaps. On the outside are signs directing people inside where they will receive free tax advice. Once inside, you find that there are television screens all around displaying explicit ads for porn sites.
Upon complaint by people who walked in and were offended - perhaps with their children - do you really think it would be an acceptable defence for the stall owners to say that they don't decide on the advertising content and leave it up to some third-party?
I apologise for the continued analogies but the simple truth is that these things have precedent in the 'real' world and the idea of holding a company responsible in situations like this is far from unusual.
The standard legal protection is to put a in disclaimer - the way video games do with warnings that the "experience may change during online play". If you gave out a free DVD advertised as having childrens' cartoons on it and then, half-way through, an advertisement for an R-rated movie - complete with 'violence', 'nudity', 'coarse language' and, of course, 'adult themes' came on, you would be right to complain. Unless, that is, the DVD was clearly marked that it may contain adult content, in which case you bloody well wouldn't pop it in the player for your 6 year old.
Likewise, if sites like this are going to rent UNREGULATED space on their pages for ads that may contain malware then they should be required to have a disclaimed displayed before you get into the site that clearly states that it contains unregulated content that may include malware served as advertisements.
The expectation is that, if you go to the Forbes site, the content on that site will be controlled and regulated by Forbes. It isn't bloody 4chan, after all.