Yeah, the red line thing is important but I agree on the no common sense sometimes. When you're a bored SF or MP person sometimes all you need some excitement to break your day up.
And if you see an old man rolling downhill in a wheelchair screaming because he can't stop and he's heading towards the red line... you lock and load I guess.
Didn't the old guy read the sign that says "Use of Deadly Force is Authorized"?