I had an interesting conversation with Anita Gardeva today about this where she expressed being puzzled over why less people chose to buy the photo when half the money went to charity and they could still pay-what-you-want. It's interesting that the social norms around charity are such that giving nothing (not participating) is seen as better as giving a token amount (and coming across as cheap).
Upon reflection, I also wonder if the people who participated in the pay-what-you-want with half-to-charity scenario now deduct that amount from their total giving by, for example, deciding against buying a chocolate bar from high school fundraisers. The researcher examined whether net spending in the amusement park was offset by the photo purchases (which it wasn't), but I would be interested to see a survey of the charitable pay-what-you-want participants to see if they offset their future charitable contributions based on that purchase. In that case, the bottom line to charity remains the same - the researcher just found the subset of people (about 5%) interested in charitable giving.