When the Ghost of Christmas Present says these words to Scrooge in stave 3, he is actually quoting Scrooge himself from earlier in the novel, in stave 1. When the two gentlemen came to Scrooge's counting house to collect money for charity, he asked them,
Are there no prisons? ... And the Union workhouses?
He is attempting to justify not providing them with a donation to help the poor. He actually says that, if the poor would rather die than go to these places, then they should "do it, and decrease the surplus population."
The spirit says these words to Scrooge now that Scrooge has begun to feel some compassion for the poor, for those who are less fortunate than he is: the spirit has shown Scrooge the Cratchit family, including Tiny Tim, for whom Scrooge has already developed a soft spot, as well as the two poor and frightening children from under the spirit's own robe, Ignorance and Want. When the spirit throws Scrooge's own words back in his face, it makes him hear how terrible they are.
Dickens uses irony here. It is ironic that Scrooge would utter these words fewer than twenty-four hours earlier, and mean them—and then, just a short while later, he would be so ashamed to think that he had said them. Hearing the Ghost of Christmas Present, such a generous and loving and joyful spirit, say these words is also ironic, and Scrooge realizes that a good person would never utter them. The changed attitude with which he meets the next ghost seems evidence of that.