They have every right to complain (whether or not they stop using Uber). They are (successfully it seems) drawing the wider public's attention to an issue. Low-skilled labour always teeters on the edge of turning into sweatshop conditions (huge working hours, poor security, low pay). This is especially an issue for monopolised industries (like Uber). I'm not sure how bad it is for them really (I live in Australia), but I really don't see how you can take issue with their right to discuss it.

As far as I knew, not being able to complain about conditions was something for China and Africa, not for advanced egalitarian societies...