The full picture is never used in academia, let me assure you of that. If it weren’t for articles like these, most people would not know where it even came from or that the model was nude in the first place. Not defending the use of the picture, strange choice of a test picture for sure, if you know where it comes from but wanted to give you some context.
If I had to guess I’d say because “an unknown man” can be intepreted as “an average man” which obviously is going to hit a lot of people.
The actual statistics of man vs bear is not really the point through, and a large number people did not get that. It’s just that the question was phrased (intentionally or unintentionally) in a way that lends itself to this comparison.