Branded as a lunatic for saying so, I've always said that it's a combination of social security and resources. The government knows they can't afford the SS bill so the simplest solution is to kill them off as quickly as possible. Also, the rich, who own the government, get nervous when the population grows and begins to strain available resources. Same solution in this case. Also helps them that they can make money off it in the process. There are some evil people in this world and it's a shame they can't be the ones to suffer rather than the innocent.