Segregation was described as wilderness as a way of indicating the inhumaneness of this treatment, for literally wilderness indicates a place without civilization or society.
Injustice is not always a wilderness, although it sometimes can be. Injustice acts within established society all the time -- sometimes, de jury, it can be under the guise of a law-abiding action, but in reality (de facto) it could be injustice. For example, Donald's Trump urge to deport and restrict immigrants from certain countries reflects injustice in an established environment (in my opinion, of course).