[shimon]

You're right, but upwork is the dominant marketplace for low-cost contract labor. It is the merger of what was previously a duopoly in the space (elance and odesk) and the network effects are strong.

So maybe it's not quite as monopolistic as the others you mentioned but it has clearly (and rationally) chosen to compromise fairness to its users to a degree that it likely could not in a more competitive market.