Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
The vast majority of computer users don't know or care about the underlying technologies that drive the gadgets and utilities they access on a daily basis. Most of these users run Windows and have little idea of what to do when things go wrong. Perhaps the drive won't boot or files are corrupted, random messages pop up, the registry or the file system is broken. The problem may be blamed on a root kit, a broken program, or a virus. Sometimes the data is lost, and sometimes the user gets lucky with a rescue disk supplied by one or other of the anti-virus vendors.
As often as not these rescue disks will boot a version of Linux. For instance, the Kaspersky Labs rescue disk runs a version of Gentoo, Panda Security's SafeDisk is based on Debian GNU/Linux, and BitDefender and F-Secure are based on Knoppix; and these are not the only examples.
The anti-virus vendors use Linux for practical reasons. Linux belongs to no-one and is free, but cost is not the decisive factor.