I don't know how specifically on-topic this is, but it has been my opinion that affiliate programs have been the best DRM protection ever. If you go looking for free porn on the web you are misled by one hungry webmaster after another offering you free porn and giving you only teasers until their will is broken and they pay.
By giving webmasters custom watermarked content to distribute over p2p networks, porn companies could saturate the p2p free porn searchers with promo clips, thus making it harder to find free porn.