It may be that they are recording it for stas purposes. If a person is stopped it could be because of his or her appearance and if an individual is stopping a higher percentage than would be the norm then there may need to be isues addressed.
I am white but you can not tell from that if I am French or German for exampe so if someone is biased against a particular skin colour, there is little point asking for the nationality is there?
There are also folk who may be of mixed race and it is for them to say what they class themsels as.