In a society with such a gender imbalance, women can be more picky and some men are left without hope of getting a partner. Without a partner, what are they going to focus on life, if not work? And who is gonna get their money when they die?
Communists may be evil, but they aren't stupid, so it's unlikely they haven't foreseen this.
I'm in Cambodia right now. Communists are, in fact, quite stupid in addition to being evil. However, that doesn't really pertain to the infanticide thing, which is more a characteristic of very poor areas than communism.
EDIT: Did the math and concluded that for a normal sex ratio of ~103 to turn into one of 120, you'd have you be losing 14 of every 100 females through either prenatal abortion or unreported killing at birth.
All in all I'd have to say this helps explain why the PRC is ramping up their police state (e.g. see http://online.wsj.com/article/SB1000142405274870424050457458...)