positive discrimination is usually seen as discrimination with an ethical, and therefore justifiable, purpose. For instance, African Americans were long held back by being barred from education, jobs, etc. When they were granted full civil rights, 'equal opportunity' was not enough - they would still be competing for jobs agaiinst whites who'd had the advantage of the education and experience denied to blacks. So positive discrimination would be permitted, to give them a leg-up so to speak, or (another metaphor) to try to level the playing field.