Did you the article? Serious malware on the mac.
1. AV program catches the attempt, prevents the installation.
2. Much simpler from Apple's perspective: prevent installation from outside the App Store.
"I have never found more than one or two in-the-wild reports. This time, the volume is truly exceptional."
I went to the forums and searched for Mac Defender, Apple Security, and Mac Protector and found a few dozen posts. Plenty of people posting straightforward solutions.
Definitely a nice piece of linkbait.