Maybe they're just trying to catch the people who don't know better.

The ones who do know better would just compare multiple documents in an attempt to pull out the common bits and remove the bits with encoded information, or use some scheme that would mangle the encoded information if they only have a single copy of the document. That would make those people harder to catch, so why bother going to the expense of trying to catch them if they can drive piracy far enough underground by discouraging the naive pirates?