Immigration reform is notoriously contentious. Yet it's hard to find anyone who doesn't think employers should be barred from hiring illegal immigrants — and sharply penalized if they do so. When President Obama last week endorsed what he called the "common-sense, bipartisan" immigration package now before the Senate, he praised it for getting tough on "shady employers" who "knowingly hire undocumented workers." The White House website devotes an entire page to workplace enforcement of immigration restrictions. It leads off with the president's call for "cracking down more forcefully" on companies that employ undocumented aliens, and boasts that his administration has...