Use common sense, practice safe computing and know why you are using your administrator password
This statement is a bit misleading, and incorrect.
...no administrator password is required to install at least two known variants of Mac malware.
It's not misleading or incorrect. Using common sense and practicing safe computing (such as not installing software that you didn't intentionally research and download from a reputable site) will avoid the malware you mentioned, whether the admin password is required or not.
... purchases of three of the most popular fake AVs
No antivirus can protect users from stupid actions, such as entering your credit card information for the installation of some unknown software that asked you to install it.
Many new Mac users are not as smart, computer literate or experienced as you are.
That's why they come to a forum like this: to learn how to be more computer literate. That's why we give them the information that we do, to better equip them to practice safe, responsible computing.
I would suggest to any Mac user that having protection against the dangers of the Internet is the safest way to easily avoid Mac security risks.
Of course you would suggest that... you want to sell your software. And it's NOT the safest way. When MacDefender first showed up, no antivirus app identified it as a threat, including yours. Those who practiced safe computing were completely protected. The safest way to easily avoid malware is to educate yourself, use common sense, and practice safe computing.... NOT to depend on 3rd party software to protect you.