"That is not fair."
Translation:Eso no es justo.
Neuter is often used for abstract things, situations, unknowns. In this case, the "whatever it is," the "eso," is said to NOT be justo, so there is no need for the demonstrative pronoun to agree with "justo," anyway. Someone may be seeing or reading about an unfair situation and says "That is not just = "Eso no es justo."