So you are saying that OS X virus software was written to examine and defeat code written for Windows?
These days, that's about the long and the short of it. E-mail viruses rely on Windows to auto-execute attachments (they were usually VBScript or something basic that Outlook loved to execute automatically). You cannot get e-mail viruses easily anymore as all current versions of e-mail programs have stopped auto-executing code, but there is always the chance that it could be passed on to someone who is still vulnerable.
Word macro viruses are another example of this type of virus. Not very sophisticated, and rely on some stupid design by Microsoft to auto-execute untrusted code in
documents as if nothing could possibly go wrong.
In the case of the iPhone OS, the apps are sandboxed for the most part. If an app does become infected, that infection cannot get outside the sandbox to infect other applications. If you sell an "anti-virus" app for the iPhone, it will also be sandboxed and cannot actually scan anything other than itself. Not very effective.
There are obviously exceptions here. Jailbreaking can be performed with physical access to the device (which pretty much means you are screwed anyways), and Apple apps still seem to be outside of sandboxes which is not a very good move, IMO.
Antivirus could be used on a jailbroken device, but it would sit there burning battery life scanning for all of one virus that can be mitigated simply by changing the root password after jailbreaking, and only affects jailbroken devices.
Put simply... anti-virus cannot function unless Apple writes it into the firmware on official devices, and jailbroken devices have a whole whopping one worm that can be defeated by practicing proper security. There isn't anything for AV software to do right now other than drain your battery.