What paradox of social injustice is presented in these lines from Countee Cullen's “From the Dark Tower”? We shall not always plant while others reap The golden increment of bursting fruit, Not always

What paradox of social injustice is presented in these lines from Countee Cullen's “From the Dark Tower”?

We shall not always plant while others reapThe golden increment of bursting fruit,Not always countenance, abject and muteThat lesser men should hold their brothers cheap;