I think it's cause of the mail harvester programs.
Example, if i wanted to spam a book on cars, I would extract addresses from car websites, which would work to some extent.
Doesn't work for porn though, cause porn webmasters aint gonna be a good target market. The only way it would work, is to extract addresses from forums and stuff where porn SURFERS hang out.
Disregard this if I didn't understand your question correctly.
