If you read the actual this press release describes ( http://arxiv.org/abs/1007.2327v2 ), it explains that the "Top Publishers" usually try to promote the URLs of their own websites. This can be done by adding the URL to the filenames in the torrents (such as HarryPotter-slashdot.org.avi ) or in the metadata on the portal. The publisher makes profit from ads or subscriptions to the site they own, not from ads on the indexing sites.

The 100 p2p users basically refers to 100 unique IPs that were responsible for creating and initially seeding the torrents; they aren't necessarily related to the people who actually first provide the content.

They acknowledge that some of the publishers were altruistic people who published legitimate files without trying to promote other sites, but these individuals were less prolific than the ones promoting other sites or distributing fake content.

Google gives the most results for "dogs chase cats"; that must be the correct answer.
For something like this to work, the phrases would need to be uncommon in a standard corpus of text. (Yet still be easily understandable for a human)