The gig economy is shining a new light on old questions about how workers are classified that Congress—not the Labor Department—will have to resolve, Labor Secretary Alex Acosta said Nov. 9.

“We’re a democracy, which means that Congress should take the lead,” Acosta said at an American Bar Association event. “It’s very easy for the executive branch through guidance documents to say ‘this is what we think,’ but that’s not the way democracy works. You don’t make laws through executive...