But shouldn't be, if the act that got the mother into the US in the first place was illegal.
It should be like robbing a bank and giving the money to your mother. Even if the money is in her account and she hasn't broken a single law, the money still isn't hers if you get caught.
The US is kind of a "blame not the child for the transgressions of the father" type of place. We decided long ago that anyone born on our soil would be an American. It isn't the kid's fault that the mother broke immigration laws. The bank robber mother could choose not to take the money. A child cant choose where he is born. That child (born on American soil, born into the American legacy, and all that patriotic mugawuga) is just as "American" as you and I and is not "illegal."