Those are racially motivated acts if they would cuff a black child, but not a white child. So if you're accusing them of refusing to cuff a white girl while cuffing a black girl, you're accusing them of being racists.
You word this as if it was a concious decision on the part of the police. I think what he is saying is that it just wouldn't have happened that way if the girl was white. I seriously doubt the cops think "She is black, and I hate blacks, so I will cuff her because I hate blacks!" There is a racial bias in each and every one of us, (well, most of us at least) including the police. It's slight in some people, and it's blatantly obvious in others. That racial bias can and will affect judgement, at least initially. The cops may not have cuffed a white girl because they can associate more with a white girl than they can a black girl.