Carrying cash is good for your wallet

I was surprised to read that are willing to pay more when they pay by credit card. Apparently they actually have a higher willingness-to-pay if a credit card is used, relative to a cash payment. Freakonomics reports an instance where a beggar made more money than ever by just holding a sign which solicited credit card donations online!

I’m baffled as to why people would pay more by credit than by cash. Does spending just feel more tangible when you have to hand over a wad of cash? If so, I wonder if EFTPOS encourages more spending than cash!?