The Cardinals are down 5-0, which is something that should make the opposing team nervous, of course. But Mike Matheny isn't managing the opposing team; he's managing the team that's down 5-0, and he's making moves accordingly. Or, rather, he's not making moves accordingly.
After Carpenter allowed four runs in the second inning, his spot in the order came up in the top of the third. Joe Kelly had been warming in the bullpen for the Cards, and he was likely ready, but Carpenter hit for himself, and Kelly sat down.
The knee-jerk move would have been to pull Carpenter, but that would have meant the Cardinals' long reliever wouldn't be available for a Game 7, and Kyle Lohse was shaky with his command in a Game 3 win. Keeping Carpenter in isn't conceding the game; it's an acknowledgement that if not for an error, he would have escaped the second with minimal damage, and that the Cardinals don't have the benefit of an off day to rest a chewed-up bullpen.
Carpenter made Matheny look smart in the bottom of the third, striking out the side. The Giants are still up 5-0 in Game 6, but Carpenter looks like he's settling in.