VALVe themselves have game IPs they want to protect. Within the boundary of "it's legal, wygd" there are always ways to make pirating VALVe games easier and VALVe will stand no ground to ban those.
I think what they sells are more about services that support the game. For example: you can find l4d2 crack easily. But you are never able to join to the official community server list without a proper purchase.
When you are confident in your own products, you don't have to stoop to cheap user-hostile bullying tactics to protect your wares.