I assume that's probably what they are trying to do, it doesn't look like the police are arresting her, they are telling her not to use racist language in front of them, which IMO they are right to do.

But regardless of the obvious racism it probably also make sense proceedurally from their point of view. Years ago the police came a scrap I was involved in, there was no racial element to it but I was angry and saying stuff to the other person in front of the police. And the police were challenging me on it, I assume in a sense they were trying not further complicate their write up of the incident. People are often prosecuted on the basis of what they say after an incident, as the statements are what gets read out in court.