Is Microsoft responsible for spam?

Fact: Spam made up 94% of all email in December, 2006.

Fact: More than 80% of spam comes from "zombie" machines.

Fact: Zombie machines are predominantly Windows machines. Microsoft's own figures state that 62% of all Windows machines they scanned are "zombie" machines.

Microsoft may not have created the spam problem we are facing but their insecure operating systems are the primary mechanism by which spammers are sending spam. Isn't Microsoft liable for this? Why aren't they being held accountable for it?