People aren't prone to change unless under pain and hardship. When people are comfortable, they remain as they are, even sitting, right until the moment before the explosion, because the explosion hasn't happened yet. People will just keep doing what they're doing, because they're comfortable, even if heading right toward a cliff.

This problem is exacerbated because Western peoples have been so comfortable for so long, that our survival instincts have been dulled a great deal. We rely on others to save us, and face almost no dangers ourselves.

Every good self defense course will tell you to remain at a heightened alert, to watch what people do, so you are prepared for the attack and react to it before it happens. Most people aren't like that. Again, their survival instincts have been dulled.

Thus, you have situations exactly like this. Even a moderately aware person, even assuming it could be a joke, would have run for their lives and/or tried to save/warn others, or stopped the assailant, immediately upon seeing grenades being pulled from that man's pockets. To do anything less is to invite death.