“Allow non-intrusive advertising” option is enabled by default for all users but the ones using privacy filter lists (EasyPrivacy, Fanboy’s Tracking, Adverisity Privacy).
Palant goes on to explain the move in a separate document:
Starting with Adblock Plus 2.0 you can allow some of the advertising that is considered not annoying. By doing this you support websites that rely on advertising but choose to do it in a non-intrusive way. And you give these websites an advantage over their competition which encourages other websites to use non-intrusive advertising as well. In the long term the web will become a better place for everybody, not only Adblock Plus users. Without this feature we run the danger that increasing Adblock Plus usage will make small websites unsustainable.
This setting will be turned on for all Adblock Plus users. Why? Here's why:
Why is this feature enabled by default?
Because that's unfortunately the only way to reach the goals outlined above. If we ask users to enable this feature then most of them won't do it — simply because they never change any settings unless absolutely necessary. However, advertisers will only be interested in switching to better ways of advertising if the majority of Adblock Plus users has this feature enabled.