Aside from reflexive ass-covering, which is to be expected; Yahoo(and any of their ilk in the advertisement slinging business) have a fairly obvious incentive to deny the seriousness of the problem.
Ad networks are a ghastly open sewer of shoddily vetted and frequently dangerous crap; usually served agonizingly slowly and heavy on Flash and scripts and crap. Even better, ads offer a nice way to hit a broad selection of users, across sites, and without needing to compromise specific operators or lure people into the seedy side of the internet where people stereotypically go to get unpleasant viruses.
Even if you are one of the 'But advertising experiences enable the content economy, ad-blockers are immoral and killing businesses, etc.' people, what do you say about the sheer danger? Leaving ads unblocked is about as safe as letting sewage into your drinking water distribution system. That's a problem. Fix your ghastly excuse for a platform, so I could at least let my guard down without getting cyber-syphilis, and then maybe we can have a chat about whether ads are wonderful or not. Until that time, don't even bother.