The parents don't immigrate to the US. They stay long enough to have the child, then they go back to their home country.
The child grows up with an option to move to the US without any immigration hassle; and, being "natural born" (which apparently does not require a person's parents to be US, nor does it require being raised in the US), is not ineligible for the presidency.
It also makes it easier for the parents to “escape” China under chain migration rules.