A hammer can be so easily misused in that you can try to knock in nails using the claw side.
How come nobody is telling them to use the other side? If the hammer doesnt have instructions on the side to say to use the hammer side, then it's being used exactly as intended.
I have seen here and there people try to use the wrong side of a hammer, usually either small children or people with no experience at all using hammers. But the second you show them the right way it sticks: the sides are visually different and, more important, the right side works better.
What I've never seen in my life is a random group of 9 people (per the study) where all of them consistently use the wrong side of the hammer repeatedly across four sessions.
How come nobody is telling them to use the other side? If the hammer doesnt have instructions on the side to say to use the hammer side, then it's being used exactly as intended.