You do not need to apply for US citizen to be a legal immigrant. The first step in acquiring US citizenship is acquiring permanent resident visa, also commonly called Green Card. Immigrants with a green card are legal immigrants and can stay in US indefinitely. My best friend in Chicago was a legal immigrant from Germany and for some reason never applied to be naturalized a US citizen.
So the distinction must be addressed between a legal immigrant without US citizenship and an ILLEGAL immigrant who is breaking the law.
The point here is that children born to foreign nationals in the United States, whether here legally or illegally, are not United States citizens.