Because when you murder a cop, you are attacking ALL of us... ALL of society. You are murdering a man, but you are also going after our government element that is intended to protect us all. Killing one strips away a portion of EVERYONE’S rights and wellbeing. That has always been the thinking.
That same argument could be made for “ordinary” citizens, too.
L
I got to experience harassment by cops for twenty years until the guy who actually did the crime got caught.
Equal protection under the law, elevating one group over any other is wholly against the very foundations of our system.
The penalty should be the same.
Currently one can get away with murdering you and I due to being a member of a preferred protected class, i.e. see illegal aliens killing and getting away with it.