Because if he judges it to be synical attempt at stopping Torres from getting into the box for a possible goal scoring oppertunity its a yellow as opposed to a red for denying a definate goal scoring oppertunity.
Well he didn't give a free kick because spain ended up scoring anyway but I wouldn't have minded if he had done. Assuming there was contact then Spain deserve a free kick but it's also clearly accidental and Torres mostly trips over himself. Hence no yellow card.
If spain hadn't scored and the ref had given a free kick without a yellow, I doubt anyone would have blinked. Hell, the same player did a much worse tackle without getting a yellow earlier.
I'm not going along with the idea that if was either no foul or a yellow.