This is a pretty loaded question, but I will go ahead and respond to it. First off, some people cannot afford birth control. Why can't they just not have sex? Well, you try going without sex... We certainly need to provide more affordable birth control for impoverished peoples. We also need to properly educate them on how to optimally use birth control.
Secondly, many of our greatest minds have come from impoverished homes. I am not trying to say that should be a reason for people to make babies like rabbits, but it is a potential silver lining to having a child born of that type of situation.
Lastly, some of these people were not poor when they had their children. Job-loss, illness, death, or any number of things could have led these people to their socioeconomic status. They likely never intended on raising their children in the lifestyle that they currently are being forced to lead, but they are doing the best they can with what they have.
Before jumping to conclusions, perhaps we should look at the bigger picture of WHY they are poor, WHY they lack proper education (which is not at all their fault, but society's fault), WHY they lack proper medical care (again, society), and all of the other WHY's.