The Western definition of "injustice" is irrelevant to the discussion. To interpret the passage, you must rely on the Islamic definition of injustice.
Violation of a person's rights is injustice. Rape is injustice because it is a violation of a woman's right to the control of her body. HOWEVER, a slave woman has no such rights. Her body is the property of her Muslim master. Under Islam, only fellow Muslims have equal rights with Muslims.
Does a child have rights? A Muslim lunatic can “marry” a five year old child against her consent and rape her to death. It’s not rape because he is allowed to marry the child, have sex with her, and kill her with sex.