Earlier quoted context omitted.
Because it's not just about showing an ad to the user, it's about knowing which user to show that ad. The ad networks track you across the web, and in the process learn a great deal about your demographics and interests. This alows businesses to target you specifically. Spiegel can only track you across their own site which might not tell them much about you, making the ad space less valuable.
You're right that it doesn't work for the advertisers. It also isn't a technical solution: ad-blockers don't just use domain names; they also block based on tags, ids and css classes, etc..
and tags, so I don't understand your second assertion.