Imagine the situation. You are in a crowded shopping street, with department stores and brand shops all round (like in Ginza in Tokyo, the 5th Avenue in NY, Oxford Street in London, the big boulevards in Paris...). Suddenly a man snatches a woman's brand bag (probably with her wallet, etc. inside) and starts running away. You were justly strolling in the area and the bag-snatcher is running toward you. If you don't do anything, he will run past you and escape. What do you do ?

Depending on my mood and the people around, I would either not care or feign indifference then knock the guy down as he runs past me.