One of my non runner friends watched the videos and asked me how people could have kept running toward the finish even after the explosion. But, I totally get it. You've spent the last 26.2 miles thinking of nothing but crossing that finish line. It's the only thing on your mind, you have no idea what's going on around you. You are depleted of just about every nutrient, fluid, and oxygen. You're lucky to even know your name. There's no way you'd be of sound enough mind to process "oh that was a bomb, I better get out of here". The best you'd come up with was "wow, I'm really losing it, I could have sworn that was a bomb".
One of my friends was 1/4 mile from the finish and got pulled... I'm guessing she's not that upset about not having gotten the opportunity to be at the finish.