ParasiteWare sucks! What is being discussed here is officially known as "Cookie Stuffing".
Check this site out for more info on this issue:
http://www.benedelman.org/
Many mainstream companies like ourselves oppose this heavily and do not tolerate it. However, sadly there are still a large number of companies that just plain and simple do not care and are only concerned with the bottom line.
AffiliateFairPlay is a great company. Kellie aka Ms. B is very knowledgeable in this. I sat in on a lecture she gave. Really informative!
-=Chipmunk=-