Running Up the Score

Why is scoring a lot of points sometimes bad? I read this morning how, when the Minnesota Viking were up 27-3 with less than two minutes left, they scored another touchdown, and were agressively criticized by a Dallas Cowboys player for "running up the score". Isn't that the whole point of the game? To score as many points as you can while preventing the other team from scoring any? Should the Vikings have let the Cowboys score one of their own so that they'd feel better about themselves?