Justice is due them, kindness and benevolence are due them, from the citizens of this happy country.
Can the 'due them' be replaced by 'due to them'? Can we use due as a preposition?
Due to them can certainly be substituted for due them in both cases.
I'd like to issue a wild speculation here and see what people think. I suspect that due them is more solemn than due to them. I have absolutely no evidence for this; it's just what my heart tells me.