That becomes a business decision! My sister's father in law was a legal immigrant from Portugal, he worked hard all his life, but he could hardly speak English, his kids do speak English.
But none of this answers the question asked, "Why should it be an employer’s responsibility to enforce immigration laws?".
Refusing to hire an illegal is not enforcing the immigration laws. I suspect you know someone that has illegals working for them. It is a moral issue at the very least.