Why? Because they're civilians? Because they're cops? GTA, anyone? Skyrim?
What is the difference between playing Diablo 3 and killing literal millions of pixels whereas this is killing literal millions of pixels? Because they're not portrayed as "enemies"?
What defines enemies? You're told to kill them? By magical glowy guy in the sky? Orders over the radio?
I apply my own ethics to most of the RPG and shooters I play, why? because I have a tendency to get very much emmersed into the game world, this gave me some issues with modern warfare 2 during the airport scene, you can call it one of my personal flaws if you wish I dont realy care.
I never played GTA, but in Skyrim and its predecessors I don't attack every random person that I come across, and try the most "moraly right" choices I can while playing them and other games that gives you different moral choices.
this games objective is practically and literally to kill as many Innocent civilians and bystanders as possible for as far as I have heard and read.
games like GTA, and Skyrim have a storyline to follow that does not general include killing civilians, innocent people(or NPCs) even if there are some innocent cassualties sometimes. otherwise, yes you can chose to go on a killing spree in most sandbox games, but it does't do a whole lot to progress the storyline.
as I said, maybe other people are attracted to this game, if a close friend would say he enjoys this game after buying it, I wouldn't judge him any different from normal. everyone is different.