And a product produced in a state, that doesn't cross a state line, is not involved in interstate commerce, either, but the commerce clause has been used to regulate many things based on the notion that, even if a product crosses no state line, it affects interstate trade by failing to do so.

In this case, however, you're wrong. You are free to contract with the citizen of another country to do whatever you desire -- but in entering this country, that person has brought the traded commodity, his labor, across the international boundaries of the United States, and the import of that commodity can be regulated under U.S. customs and immigration authority -- authority that is delegated to the federal government in the Constitution.