The 'the horses wouldnt exist without horse racing' line, to which you allude, is a dubious defence.
Would you agree with breeding humans just to be slaves? They should just enjoy it because they wouldnt exist without it right?
I mean its not like these horses spend 90% of their lives reined (chained) up and 80% of their lives in a small cell?
Born to be caged and chained up?