Underlying principle: monopoly of union balances monopsony of employers

My caveat about one concept in a great, comprehensive survey:

Unions in the Precarious Economy, How collective bargaining can help gig and on-demand workers
Katherine V.W. Stone February 21, 2017

“If gig workers in the United States are found to be independent contractors rather than employees, they cannot benefit from federal and state laws guaranteeing” … “as independent contractors, any collective action they might take to change their working conditions could make them liable for antitrust violations.”
http://prospect.org/article/unions-precarious-economy-0

I’m thinking, UPS and FedEx are identical companies (UPS 550 planes, 60,000 trucks; FedEx 650 planes, 45,000 trucks). If the First Amendment guarantees the right of UPS truckers to collectively bargain, how to justify denying the same right is possessed by FedEx Express drivers -- even if they had been (are) found to be individual contractors, not employees.

Underlying principle: monopoly of unions needed to balance the monopsony of employers. Country founded on balancing competing interests.