What is the tone with which Sinclair inserts the line, "So wrote a poet, to whom the world had dealt its justice," between the stanzas of the poem at the end of the chapter?

This makes sense if you examine the stanza,

"The vilest deeds, like poison weeds, Bloom well in prison air; It is only what is good in Man That wastes and withers there; Pale Anguish keeps the heavy gate, And the Warder is Despair.

So wrote a poet, to whom the world had dealt its justice—

I know not whether Laws be right, Or whether Laws be wrong; All that we know who lie in gaol Is that the wall is strong. "

Here the poem refers to the injustices Jurgis feels every day. He realizes that laws are not there to protect him, they exist to keep him down. They exist to force him to be a slave to the companies. He feels like he is in prison and, while others profit. he slowly dies.