Freedom of choice comes in degrees, with no sharp divide between free and not free.
The freedom of an individual to negotiate with employers exists in a context of wild inequality in negotiating position, information asymmetry, and coordination problems among workers that lead to a race to the bottom (absent collective bargaining or government intervention).
I don't know enough about Uber and their workers to comment on that in particular, but I don't think that "they could have not signed on, so it's OK" is necessarily a strong position from which to oppose collective intervention, governmental or otherwise, without qualification of the degree to which participants in an agreement were free to make their decisions, and what the consequences of thier decisions will be on the choices available to others. Since we're talking about low-wage workers and a giant company, my guess is that closer examination of those considerations won't aid an anti-intervention or pro-market (for common definitions of pro-market) argument.