I think they do it of pure emergency and fear to create a demographic monster. But I think if they just left it to the people themselves to get one or more babies or not, it would not mean the population would explode in a chaotic way. And China has all means to expand: There is pretty much space in the West for China to expand (Kirghizstan, Tajikistan etc...).
But making a millionfold murder on non-born lives can NEVER be excused. Not even if the population of the country hit 3 billion. I am pretty sure there would be a solution for everything. In Europe we would be happy if women actually got more children! So, if China has too many of them, sned them here to Europe. They could breed out the Muslims and they are mostly very well integrated and successful.
BTW: The East Asians and Africans are outbreeding the Muslims here in Germany! The Muslims' fertility rate dropped to only 1.7 children. While Asians and Africans still top 2.3!!!
German women don't want to have children, it seems: 1.4 children...