I think you're thinking of this in too simplistic of terms, like they just fear that someone will say 'you're racist'.
But it's nowhere near as simple,these people lack integrity, they don't want to rock the boat, they follow the culture rather than their oath, they take the easy way out, they worry about the consequences for themselves rather than doing the right thing. If they know they're not being racist but fear that they will look racist or that someone will call them racist to shut them down they just do nothing.
To me it is an explanation because many people are like that. Many people are followers and sheep, many people lack integrity etc unfortunately that is just how many people are. It's not an exciting explanation, it's just how a lot of people are unfortunately.
As I said I don't think a lack of integrity excuses a person in any way even if the majority of other people also lacked integrity.
Another aspect of it is that they were apparently concerned about causing racial tension, with that added aspect they could tell themselves they were actually doing the right thing, even if they wanted to help these girls the consequences for society would be dire so they essentially let the girls become collateral damage.
I think it's different if it's an individual crime rather than a widespread organised crime gang and they would have felt far more confident about doing their jobs when it came to investigating other crimes, that was far less likely to cause unrest in the area and if you're apprehending one suspect as opposed to saying there's gangs and gangs of these men from a certain country then people are far less likely to be trying to shut it down by making out it's 'racism'.