Most of these photos got sold through auction. Peter Lik reportedly sold a photograph titled Phantom to an anonymous bidder. At $6.5 million, it is potentially the highest price paid.
I remember watching a TV series where a famous painting got stolen from a museum (I can't quite remember the name). There is a sentimental touch around art, and now that you've mentioned it, I see why someone would choose to remain anonymous.Reduces the chance of theft I would imagine. Art theft is a thing and the thieves are very high tech about it. Strangers don't need to know who it was.