Most of the virus companies sell Virus software for Linux. But not for the reasons you might think. Think about Samba file servers. Linux can scan the Samba shares for Windows viruses and clean them up. Also for the FTP sites who must make sure they are providing virus free software. And for the SMTP servers who wish to scan email attachments before they get to their Exchange Servers and Windows end users or for outgoing messages....
In addition to be able to scan and repair Windows files from Linux machines some put in checks for known Linux trojans etc... You're right, for a Linux desktop it's a waste of time (unless yer plannin' on running that Lindows crap).