Ok when we situation when Team A fight with Team B, from positions on screen, to fight without agro npc they have to move to tight spot on chruch corner, and if they do that, is chance to be pushed to agro zone by nade/rocket luncher.
of course they can try kill npcs, but reno is mostly small team vs small team so if team A start killing mobs, team b will use that and push them before slags will die. Also slags blocking a north sewer entrance. i know that npc is easy to kill but it take time/ap/hp and decide to kill slags can effect to lose the fight, also planting c4 is difficult.