What's a "born citizen"? It's degsme favorite expression (and often capitalized), yet I'm struggling to understand what it means. Are there citizens who are not born? Are there robot citizens?

Presumably your "right to freely associate" with whomever you please has limits when it comes to employment. Employers are expected to meet certain requirements (safety, work time, payment of payroll taxes, etc.). There is no argument to say that the first amendment somehow means employers could never require proof of legal residence. They already require employees to have a social security number. Requiring proof from everyone is by definition non discriminatory. And all the crazies screaming about "illegal immigrants" should really shut up unless they're willing to scream even louder about "illegal employers".