In Western culture ambiguity can often be an uncomfortable (or curiosity-invoking) experience for us. We generally automatically assess and decode situations and expressions based on cultural knowledge and our personal experiences. Without enough information we are forced to speculate.
Typically we use context and subject information (body language, actual words, intonation, facial expressions, etc.) in our human interactions. Obviously words and intonation are absent in still images, and in this case, any cues typically evident from the eyes or facial expressions are missing as well.
These photos are an experiment to test how people will (or will not) determine, interpret and/or project emotion in the absence of facial cues--increasing the potential ambiguity. These images were purposefully framed as they are, not cropped to remove heads.