To put it simple: GangRPers choose to kill / major people. Officers are ICly and OOCly (for the most part) forced to arrest people. If you wish to develop your character in the GangRP community, you have ways to avoid major / KPS, but the ideal of "arresting should result in kps" completely erases any sort of attachment to one's police character due to the extremely high risk of non-developped characters aiming to completely ruin a player's fun by trying to rip off pages of their character's lore. I'm not saying police should be indestructible, of course. In fact, if we decide to use our guns to potentially kill your characters (which, by the way, we're also forced to treat you as far as possible), you do gain KPS in return.
TL;DR - Stop thinking that GangRP is some sort of game to see who can rip off the most pages of people's hard work and please try and treat it as a way of complex roleplay that all parties can potentially have fun with (even without consenting)