According to this, Google doesn't allow ads served by AdSense to open in a new window:
Generally, it is our policy to not allow Google ads to open in a new window. However, in a few markets, including China, we've found that opening ads in a new window better aligns with expected internet behaviour and provides a more consistent, intuitive user experience.
Keeping in mind the importance of the user experience, we've enabled Google ads to open in new windows in locations where this is expected browsing behaviour. This functionality is determined by the IP of the user, and is not an option that you can select your AdSense account.
This seems quite a significant caveat for a webmaster using AdSense, as it means any advertisements they serve on their site will take visitors away from it.
I can't imagine what Google's motive is for doing this, as not only do users generally dislike ads that disrupt their browsing experience, they're also more likely to simply return to their last-used tab/window after following an ad rather clicking the Back button on their browser once they've done so.
What reasons would Google have to do this, and is anyone aware of more information on which regions/IP address ranges Google has decided to enable the behaviour of opening AdSense ads in a new window for?