Let's suppose that there is a very stupid law, for example, one of the following:
1. A law that says only women are permitted to carry heavy loads. Men are forbidden to lift any burden heavier than 20 pounds.
2. A privacy law that forbids anyone from entering anyone else's house.
3. A law that legalizes rape on the second Tuesday of every month.
Now, one could argue against any of these laws from a particular meta-ethical perspective. One might take a Rawlsian view, and say that no one ignorant of his condition in life would choose, ex ante, a world in which men are forbidden to carry heavy loads. One might argue from the perspective of natural-law libertarianism that for rape ever to be legal, even just one day per month, is a violation of human rights. One might argue from the perspective of Aristotelian virtue ethics that forbidding private hospitality impedes the cultivation of the virtues of loyalty and gratitude and makes it harder for people to fulfill their telos.
Against this, a defender of one of these laws might say: "OK, maybe the law is unacceptable from your particular meta-ethical perspective, but why should we agree that your meta-ethical perspective is the right one? I have a different meta-ethical view than you, so we'll just have to agree to disagree." Why would such laws have any defenders? Well, for one thing, there may be people with an interest in maintaining them. A certain kind of man might like law (3). Proprietors of open-air restaurants could benefit from the prohibition of domestic hospitality. So there is a need to argue, to bring things onto the plane of principle.
It might be an effective persuasive technique, in this case, to up the ante by saying that you can show the law is wrong from any meta-ethical perspective. Of course, that can't exactly be strictly true. If your meta-ethical view is that the only desirable thing in the world is male laziness, you might regard law (1) as a good idea. But no one holds that meta-ethical view, it's absurd. So your next step would be to take the standpoint of all the major meta-ethical views on the market, and argue against the silly law from those perspectives. Then you would challenge your interlocutor to propose other meta-ethical views, and you would either (a) dismiss the meta-ethical view as completely untenable, or (b) show how it was inconsistent with the silly law.
Other laws, meanwhile, would be defensible from some meta-ethical perspectives and not others. For example, a law permitting the torture of the innocent relatives of known terrorists to extract information about imminent attacks might be clearly unacceptable to a believer in human rights, yet clearly desirable, if used sparingly, from a utilitarian perspective.
I would claim that US immigration policy is indefensible from any valid meta-ethical perspective.