It could be because its not completely written out how these things will occur. You would assume it would be caused by similiar things in real life but there is no definate definition of what will cause a "heart attack".
I'm sure that a broken leg is something you can prevent by NOT FALLING OFF OF THINGS.....a heartattack is random...and you can't control it....yeaaaaaaaaaaaaaa so how can you compare the two? Please try again B)