His Meditation

[busker]

a poem is not a lecture, and it should not feel like one.

I think this comment nails it.
The problem with explicit pedagogy in poetry is that there isn't enough room for a proper exposition. I'm sure you could write a book about Abe Lincoln's ambivalence towards the blacks, with a whole bunch of arguments and supporting facts. You don't have that luxury in a poem.

The other problem is that there's a lot of jumping to conclusions going on behind the poem, such as the thinking that the Reconstruction basically failed because of the Republicans and not the fact that Southern culture wasn't totally stamped out and obliterated, like you would with any normal enemy (Nazi Germany, Imperial Japan), and the lands resettled wholesale with non-English speaking newcomers from Europe. It could easily be argued that Lincoln was too soft and failed, like all victors who've been too soft. Any half decent book would cover that angle, regardless of the final conclusion, but the poem doesn't do that, and clearly the author's thinking is also limited in this aspect. And so it all comes off as a half baked defence of a slave owning society, stripped of all the tassels and christmas stars.

Lincoln's idea for Reconstruction was to "let 'em up easy," as he put it to Grant.  After his assassination, the Radical Republicans went purely punitive instead - including rubbing ex-Confederates' noses in freedmen's rights and privileges - but they couldn't sustain it and Reconstruction basically failed.