why do they make virus scanners for osx then?
Because folks want to sell them and some folks want to buy them.
The only viruses I've seen reported in the wild on OS X are Word and Excel macro viruses. Only Microsoft could come up with the singular innovation of a cross-platform virus environment. If I got a lot of word or excel files sent to me as a matter of routine, I'd probably set something up to scan them.
The virus scanners also catch Windows viruses -- they can't run on a Mac, but a Mac user can still pass them on via uploads or mail attachments.
Same reason they're made for Linux computers, as a courtesy to Microsoft machines. Wouldn't be very sporting to pass along malware to a Windows machine that you're communicating with, now would it?
i run AVG FREE for Linux, and it found a Windows Virus on my user account last week. Located the file and nuked it (no registry to worry about, no worries about it affecting my system).