Almost always? That's a bit of an exaggeration though I don't doubt it does happen. Most Chinese workers do get paid, even the migrant workers. Indeed the laws protecting them are way better than laws in the US, though of course endemic corruption means laws are often ignored. You'd hear about the odd riot here and there where the builders didn't pay in a failed construction project but they'd have a tough time completing a project and not paying anyone. Certainly in any urban area.