Simply by practice.
The federal, state and local governments treat anchor babies as citizens. No one stopped this practice, so now it's accepted law.
Sometimes practice evolves into custom, and custom into law. Simply because no one put a stop to it.
Problem is, there already exists law against anchor babies.
Ignoring the law doesn’t nullify it... it just reduces the integrity of your country and it’s ability to be a nation of laws
May you be soon disabused of such nonsense through my prior reply to you.