Well, it became clear to me when I watched this video:
The driver does not just disable printing it also adds an extra page to the end of the document. They say it's there "just to inform the receiver", but I don't buy it. Marketing, that's what it is really all about. Every time you make and spread a .wwf file, you are making publicity for the WWF. Bottomline.
It gives me an eery feeling. Does it just "phone home" in order to check for "updates" or does it report how many digital advertisements have been spread the last week? It makes you wonder..
Update: According to GizMag the WWF has admitted it is all about viral marketing:
WWF Germany says that the campaign is meant to be viral and an extra page tagged onto each new format document will help introduce new users to the campaign and encourage awareness about how we use paper in our digital lives. If you don't want this extra page added to catalogs, official documents, CV's and so on then you'll need to choose another method of saving files and run the risk that such things may end up in a print queue somewhere.
This also proves the WWF is not too shy to bluntly lie. As I have demonstrated numerous times it is perfectly feasible to create a PDF that cannot be printed.
Update: It's not the first time WWF has launched a disastrous campaign. Remember this beauty?