Isn’t human use of animals a “tradition,” or “natural,” and therefore morally justified?

Every form of discrimination in the history of humankind uses as its defence that it is “traditional”.

That behaviour can be described as traditional has nothing to do with the behaviour being morally acceptable. An example of this is female genital mutilation in Africa. This is certainly traditional but that does not mean it is morally defendable. As for “natural”, our modern forms of animal agriculture are anything but natural and even if we were to return to a more “natural” form of agriculture the question would still remain of whether it is morally acceptable to treat animals as property. Additionally, the human capacity for rational thought and the ability to make ethical decisions is just as "natural" as our physiology.