I'm getting so tired of this blame-the-user-for-everything attitude on Slashdot, HN, and elsewhere.
Some of the blame belongs to Apple/Safari. You see, Safari's default settings allow for the automatic download and execution of "safe content".
The clever authors of MacDefender/MacProtector packaged things up so that by merely visiting a hostile web page in Safari, it causes Safari to automatically download and automatically run the MacDefender/MacProtector installer.
For example, you may be browsing Google Images. You click on one. The next thing you know, and this happens in just a second or two on a broadband connection, the installer program for MacDefender/MacProtector is running.
Now fortunately, the installer process itself is benign. If you quit the installer, you won't get infected. If you continue through the installer, however, the Trojan will be installed.
I can see how a lot of people might think Apple automatically pushed out an anti-virus, or something, and that's why the installer starting running automatically.
Feel free to blame the user all you want...right now, it seems very popular to blame "those stupid users" for everything. But in this case, Apple/Safari deserve a nontrivial share of the blame for Safari's outstandingly asinine defaults.