It is because if you are actively working either training or doing group fitness, many people will see you wearing her clothes and ask where you purchased them. Clients and students also see you an authority on all sorts of fitness related things and possibly seek out your opinions. You can be influential. So in exchange for a discount, you are, in essence, promoting her clothes.
She probably figures if you are not working, her clothes won't be exposed to very many potential customers.
I understand where she is coming from business-wise, but I also see how you would feel unhappy.