As reported in a New York Tims article http://www.nytimes.com/2015/12/15/technology/seattle-clears-the-way-for-uber-drivers-to-form-a-union.html?_r=1 and the Seattle Times, http://www.seattletimes.com/seattle-news/politics/unions-for-taxi-uber-drivers-seattle-council-votes-today/
Some commentators have focused on federal issues of illegal price fixing, and whether federal law allows independent contrators to unionize. If Uber drivers are in fact independent contractors, then the they would not be able to unionize, and could be found to be illegally price fixing. That said, these arguments presuppose the independent contractor status of the Uber drivers.
The issue here is not that simple. Washington State has adopted a test for determining if workers are properly classified as independent contractors, or ar more rightly classified as hourly employees.
The Washington Supreme Court recently held that the proper test for determining whether a worker was an employee or an independent contractor under the Minimum Wage Act (MWA) is the "economic-dependence test," which asks whether, as a matter of economic reality, the worker is economically dependent upon the alleged employer or was instead in business for himself.
It's not far-fetched to speculate that many, if not most Uber drivers, are economically dependent on Uber for their primary income. The analysis is different when dealing with a legitimate independent contractor such as a general contractor who lays kitchen tile. Under that sitation, the tile contractor is not dependent on any one job for his livlihood. Uber drivers are reliant on Uber to dispatch them to riders.
The questions of price fixing and ability to unionize under federal law avoid the necessary threshold question. That is, are Uber drivers economically dependent on Uber, and therefore properly classified as hourly employees.