It's certainly automatic in states that have the Death Penalty.

I also understand the reasoning...cops are there to uphold the law and while the value of a cop's life isn't greater than that of your average Joe...the act of taking a cop's life speaks of a certain brazeness and disregard for the laws of society. I guess they're thinking is that this takes a particularly deviant criminal mind to carry out such an act...and that police officers routinely put their lives in danger for the safety of the public and as such deserve a higher degree of 'protection' by making the penalty more severe.

I'm no fan of cops...but I can't really argue with this line of thought.