Sure it is Malware, but that doesn't mean it's not a threat to Mac users, a decent amount of Mac users are not very knowledgable when it comes to computers, I can see a lot of people going ahead with this install, why? well it says MacDefender, people could confuse it for an anti-virus software, so yeah I mean its entirely possible that someone could install this..
Anyway, it's to be expected, infact when Mac OS does become more popular I think we will clearly find viruses, malware and spyware, that day OSX will become a lot like Windows.. Even anti-viruses today for Windows are not able to get rid of every virus, you have to constantly do updates, even then theres always new viruses, and your not always going to be protected..
But I don't think that'll happen anytime soon..
Malwares should be treated as strangers. Simple.