Any platform connected to the internet is vulnerable. Linux, Windows, Mac; it doesn't matter but as has already been stated, why waste time going after 5% when you are far more likely to get results going after 95%? What does matter is taking the necessary precautions to minimise the chances.

A good example are the vulnerabilities that are now getting exploited with Firefox as it's popularity increases. The 'invulnerable' browser leaks!

As for the original question about big companies deliberately employing virus writers to target competition .. there's no need. Plenty of willing minds are already out there constructing the next crack even as I write.