Except that in real life, it isn't the police's job to prevent murder in such a direct sense either? The existence of police is supposed to deter murder because hey, if you get caught, there will be consequences. So please, explain to me how the Otherworlder Enforcers 'let murder happen on their watch' when there was no way for anyone to know that a murder would be committed in the first place.

I will agree that perhaps some locals might think racisty thoughts and wonder whether Otherworlders might help Otherworlders by making evidence disappear, but let's not forget that locals can be murderers and help each other out too. Discrimination works both ways.