Of course it all depends what you mean by "trade union".

From that link Ebbie gave: According to China's trade union law, all employees have the right to join the ACFTU, the country's sole trade union organised by the ruling Chinese Communist Party. However, they are outlawed from forming independent unions or organizing collective bargaining activities outside the ACFTU. The government's tight control over trade unions has led to widespread accusations that the ACFTU favours business and plays a greater role in attracting foreign investment into China at the expense of worker rights.

I can see why that kind of union set up would be quite acceptable to an operation such as Walmart. Help keep the workers in line.