I'm not condoning breaking into the network and stealing source code, but what financial damage did this cause to Valve? The article repeatedly refers to financial damages, but I'm not sure how that is.
Well first of all, the game appeared to be playable, so there's the piracy aspect. The Source engine is also licensed to other games. If the code is public, other engines could copy their features. Also it is very annoying to re-secure all your computers after you have been breached. Every single person has to change their password and you don't know what backdoors the guy has installed without a full wipe sometimes.
Like other commenters said, they used him as a scapegoat; he did zero damage except make poor ol' Gabe worry.