It's clear from what early testers have found that users will love this new capability, and we can expect iOS devices to quickly become faster-loading, ad-free zones within a year. They will probably also lose other annoying content like video and audio along the way. That's great for readers in the short term, but devastating for those who create and fund the content, and ultimately bad for readers, many of whose content sources will have gone out of business.
Ad blocking is not turned on by default, but like encryption it's always there. Just as iOS encryption needs a password to trigger it, iOS 9's ad blocking needs an ad-blocker app to trigger it. If you install an app like Adblocker on your iPhone or iPad, ad blocking is up and running, affecting every website you load. Because Apple requires all browsers in iOS to use its WebKit engine, any browser automatically enforces the ad blocking.
As a result, only those companies that have paid Adblocker and its ilk to have their ads displayed will have their ads visible on iOS 9. (You didn't know that ad blockers charge to whitelist specific ads? Of course they do — that's how they make their money for their "free" or low-cost service.)
Apple cites user experience as the rationale for iOS 9's ad-blocking technology. Certainly the intrusive ads that so annoy us all give cover to that claim. But let's not fool ourselves: Apple is also looking for ways to degrade archrival Google.
For the last couple years, Apple's marketing has emphasized how it doesn't mine user data and sell that information to the highest bidder, unlike Google. Now, with OS-level ad blocking it can reduce Google' ad income significantly. Because iOS devices are one of Google's biggest source of ad income, earning three times as much as Google's own, more widely used Android.
I wouldn't be at all surprised if Apple's own iAd service for ads in mobile apps (not websites) is exempt from iOS 9's ad blocking. In fact, Harvard University's Nieman Lab reports that iAd ads won't and can't be blocked by Apple's content blocker extensions. (Apple did not respond to my inquiry about how content blocking would work with app-oriented ad delivery systems such as iAd.) Even if iAd is blocked by the content blocker technology in iOS 9, Apple's primary income is from product sales, so sacrificing iAd to hurt Google's primary income stream is an easy decision.
Mobile devices are where the growth in user devices is and — more important — in where people spend their time. Blocking Google, Facebook, and other ad networks in mobile is a preemptive nuclear strike launched before the other side has enough of its own warheads.
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