Not everyone. They teach you about it in Psychology. The fight or flight response kicks to the body first so your body is ready to run or fight, but people won't necessarily respond hero style. Quite often fear will overcome the response to help and the person will freeze. That moment of being frozen gives the brain enough time to comprehend the situation. It's that moment when it comes down to bravery. The person will either overcome the fear and help, submit to the fear and run away, or in extreme cases of fear they'll just remain frozen completely unable to help.
But generally the person who jumps straight in doesn't do it out of bravery. More often it's an automatic reaction.
It's also possible that the person did think about it though. Generally a workman would have reached for the shutdown button then jumped in which is enough time to think about it. Unfortunately by that stage shutting down the blade wouldn't help 'cause the guy was already pinned.
