US based pilots are employed by CPA Ltd. - US branch located in San Francisco, CA. It's no different than Lloyds of London employing personnel in US offices. There are a few peculiarities wrt foreign registered aircraft, but the employment rules are essentially no different to those of a US registered carrier in terms of labor law and unionization. There are certainly no exceptions for USERRA, and there never has been. CPA will be held to American legal standards in every respect going forward. If they don't want to abide by those, they should close the base and move all the pilots to Hong Kong on full expat terms.