Well, I suspect there is a case of the police not wanting to be accused of willfully ignoring suspected racism and thus being racist themselves.
However, I'm not entirely convinced that he was in fact "arrested". Maybe just "brought in for questioning".
I think what happened was he was technically arrested, but released on "street bail" which allows the police to let someone go free on the spot if they agree to voluntarily attend the police station for questioning. I don't think (from what I have read) that he was ever held by the police.

