I live in Guatemala and it is true that many of the locals believe a camera will steal your soul, but that doesn´t really make sense in a museum. I suspect that originally cameras with flash were forbidden because of the possible damage to the artifacts, but later the museums and sites probably realized that they sell more postcards when people can´t take photos and decided to ban them completely. With cel phone cameras these days, though, you could technically take some shots without anyone noticing!