It would be anti-productive for such companies to produce viruses as they would only give decent/advanced and better programmers ideas as to new holes in an OS. Thus snowballing themselves with new viruses to deal with.
I have been known to write the odd piece of malicious code, but i would never use it on anyone but myself. I do it to prove i'm a reasonable programmer and that i understand what an OS is doing in certain instances.
It is a quesiton of morality when a programmer or hacker releases code for the use of destruction and corruption, sadly there are more destructive programmers than non-destructive in the virus industry.... BUT luckily there are companies which devote their time to fighting these nasty bits of code, undoing damage is much harder than causing it, and to those companies (though for the most part they charge a crap load for their products) i salute them