Why anyone is surprised at this, I can't imagine. Girls are perceived to be worth less than boys, so market forces result in the supply of girls to be constricted until it meets the demand. Sexism is the reason girls are devalued, but this practice isn't the devaluing, quite the reverse: by making girls more scarce, selectively aborting some increases the value of the ones who survive. Boys are the ones being devalued by the practice - and the ones being hurt, since it means half of them will have no opportunity to marry or have (legitimate) children.
I wonder if perhaps some of the outrage is related to the fact that abortion is the means used to achieve this end. Are some liberals not quite as comfortable with mass abortion on demand as they like to think they are? Could they perhaps have a vague unease stemming from the gut feeling that in some parts if China half the female babies are being killed?
Which would "feel" more like this situation: if half of all the baby girls born were exposed and left to die at birth, or if medical technology made it practical to pick the sex of the baby before conception instead of after? Something tells me that the latter wouldn't feel nearly as creepy. Maybe it's just me.