The reduction in outgoing spam from the U.S. is largely attibuted to work by ISPs and the infamous CAN-SPAM act. Spam has traditionally originated from large ISPs, however spammers are increasingly using compromised systems in the form of botnets to send out unwanted email. Spam is also frequently used to send out phishing scams and other various types of illegal activity.
I just thaught that i would add that most of the commercial spam i see is from the U.S and most of the phishing spam i see comes from china so these results dont surprise me.