Um. I'm not an attorney, but how do the US Chamber even have standing. They're not being harmed. They're not even remotely involved.

@1, it's in their interests to destroy unionization wherever it may come up.

@1: The Chamber of Commerce does not have to be harmed, their position is that Seattle's law is harming workers (and violating federal law), and as the U.S. Chamber of Commerce, that is their business.

I wouldn't fret too much about the US Chamber of Commerce's standing (which their own 2015 amicus brief in Spokeo, Inc. v. Robins argues against). This is a good opportunity for the 9th Circuit to put an end to employer evasion of federal and state labor laws through the use of fake "independent contractors," since the NLRB seems to be dragging its feet on the issue. As an aside, isn't it funny how federal antitrust law permits price-gouging cable and telecoms oligopolies to bleed us dry but are claimed to prevent Über and Lyft drivers (and probably housecleaners, too!) from unionizing?