Meaning?

Hi,

What is the meaning of "that couldn't be the law"in the following text?

I understand the non-return principle of the Refugee Act, the judge said to the lawyer. But what if the act has no force outside of the United States? Then couldn't the Coast Gard send back Haitians found at the sea?

The lawyer listened carefully. he thought the judge was tossing out a friendly question, giving him the chance to say that of course That couldn't be the law.

Thanks