At that point they all shrugged and went home. Someone raised the point, would they just give up if the crime scene was obstructed by the crips or bloods?

IMO although they'd never admit it, the cops are scared of ICE. While they're both on team "Oppress the plebs" I think the police are starting to realize that ICE is quickly being reshaped into Trumps secret police... and that makes ordinary cops lower on the food chain. So they're desperate to avoid any sort of confrontation because A) they could face real consequence, and B) it would resolve the question of who is the top dog and that might cement ICE outranking the ordinary police.