Steam gave me a virus?

Please read all of this before even thinking of an answer. I have just received £500 compensation from valve because there was a range root kits on their system which lead to me being hijacked (my whole pc, not just steam). Heres the question, is it illegal for valve to have had it on their system? As nowhere does it say that a root kit is included in the game. DONT TRY AND SAY THERE WASN'T A VIRUS, LIKE THE GUYS ON MY PREVIOUS QUESTION, I KNOW IT WAS FROM STEAM. I HAVE HAD A RANGE OF EMAILS AND LETTERS FROM VALVE ABOUT THE MATTER AND THAT IT WAS NOT ONLY ME. Sorry just had to do that in caps to make sure the fan boys can see it. I also know a root kit isn't really a virus but it makes it extremely easy to hack someone if they have one. Also just in case yoir wondering the root kit was from Alliance of Valiant Arms. Which the developers are trying to remove from there game. Thank you in advance. And dont say it was my fault because it wasnt, all my software was official.