Actually, there is a case to say that they are. Discrimination by category membership, instead of on a spectrum, means that candidates which have more merit are passed aside in favor of ones with lesser merit- particularly in the case of species, this can be problematic. The right of a person to be judged on their merits, if asked in abstract, would be accepted.

The only counter-case I can think of it is to say that society simply does not have the resources to discriminate (since discrimination it is) more precisely. However, even this does not entirely work out as within limits society could easily improve it's classification methods to better allow for unusual cases.