The fact that this is built into the OS and doesn't have to be separately installed, upgraded, or managed fits in perfectly with what makes OS X a great and easy to use OS.
Just another example of the difference between "safety" and "security". Houses in Kennilworth, IL need less security than houses in Rogers Park --- there are something like 192 police officers per Kennilworth township block --- and yet Kennilworth houses can be counted on to have state-of-the-art security, because they can, and state-of-the-art security is a selling point.
I share your take on antivirus software, but do note that preventing a user from installing something with a known virus on it does not have negative utility.
So many people these days just install anything without really checking what it is. I don't consider this anti-viral, more like a condom for the lazier users.