I guessing that people get defensive when others question their decisions/purchases. It also could be an issue related to one's own self-esteem or financial funds in this matter. Not everyone can buy or wishes to buy multiple platforms; it could be said that an attachment grows on the decision/purchase that was made. When others question your choices or puts them down, but you can't relate to their statements, you tend to speak out--like politics.
I don't believe Valve ever attacked the PS3 directly, but when PS3 success looked very questionable, their statement didn't help the PS3s PR. Fan boys just escalated it.
So I'm guessing that their statement had a ripple effect in the community because people were wondering why the PS3 graphics were coming out worse than the Xbox360. Myself included, felt a little sad when I saw the comparisons lacking for the PS3; it sometimes made me question my decision, but since I missed out on the PS2 and wanted blu-ray, I stopped caring about what was being said for PS3 graphics. Blu-ray was also on thin ice if I remember correctly. Oh how my the PS3 has suffered... *goes and cuddles his PS3* (Yes, I'm guilty of loving my PS3, but I don't want Sony to dominate! That would be the worst. Competition keeps prices down and me happy.)