It's my understanding that if it can be proven that one obtained a visa fraudulently, which is what these mothers are doing, they can be deported. Presumably, this deportation could hypothetically occur before their children are born, which would negate the purposes of their visit, although I suspect that to be a fairly unlikely occurrence. Even if it's fairly unlikely that it would be discovered that their visa was obtained fraudulently, and it's fairly unlikely that it would be discovered soon enough for them to even potentially be deported before having the child, why would they take that risk?
After all, the risk they're taking is fairly large. First off, they've already paid large sums of money to come here, and that's effectively wasted if they don't have their child in the United States. Second, by having the child here, there could be legal complications back in China, as AFAIK China does not allow dual citizenship, and does not look favorably upon what these mothers are doing from a societal or legal perspective. If they happen to be deported, you can be sure that the Chinese authorities would be notified of the deportation, and of the reason for the deportation. Aside from the risks, I'd suspect these mothers feel it is absolutely imperative, due to the benefits, for their child to be a U.S. citizen, and the best way to ensure that happens is to fly under the radar while they are in the United States.
As another consideration, I'd suspect that their ignorance of the efficiency (or, more accurately, extreme lack thereof) of the U.S. immigration enforcement system is deliberately cultivated by the individuals operating the birthing hotels / maternity homes in the United States. They have a pretty significant financial incentive to exaggerate the risks in order to convince mothers to stay with them.
Finally, I'd suspect that part of the reason is also due to language and/or cultural barriers, as well as loss of the mother's immediate social support network. Most of the graduate students from China that I know have serious issues conversing in English, at least about any topics outside of their field of study, and they've all apparently taken and passed the TOEFL; I'd suggest that those graduate students probably have a MUCH better handle on English than do the mothers in question. Being immersed in a world you don't completely understand, can be scary and stressful, and these women are already stressed due to their pregnancy, loss of their social support network due to their relocation, and fear of their fraudulent activity being discovered. All of these seem like fantastic reasons why they would want to be around others in the same situation that they are.