What makes you think that? Unless the law is enforced, why wouldn't porno producers send out free samples on CD-ROMs or DVDs the way AOL sends out AOL 8.0 CDs?
If it was an effective marketing tool, they would already be doing so.
And it is not illegal to send all prono through the mail. If it was, how would Playboy, Hustler, Penthouse, and the Adam & Eve style catalog companies get their product to customers? Last I checked, they use the USPS.