Your supposition is amazing its ignorance. Microsoft fixed the vulnerability being exploited by Conficker/Downadup last October - specifically, 15 Oct 08 with patch MS08-067. Not to mention that removal tools have been available from Microsoft, Symantec, Sophos, Kaspersky Labs and McAfee for almost as long. However, there are apparently ~10 million ignorant/stupid users (as well as corporate IT admins) that haven't patched their systems. So, sure, blame Microsoft for allowing a vulnerability in their OS if you want (because, you know, no other OS has any vulnerabilities whatsoever), but blame the users, too, who have failed their part of the security cycle.
At least something was being done to frustrate the virus that's out there, to help the users when they can't/won't help themselves. For all of Microsoft's vile business practices and software asshat-ery, at least they're pretty damned quick to respond to vulnerabilities such as this and fairly transparent/open in the process (at least, on the outside).