Answer: Yes it is.
Step-by-step explanation:
The National Labour Relations Act of 1935 was passed to cater for both employees and employers. It gave both parties rights and advocates for Collective bargaining. Perhaps most key about this act is it's protection of the rights of employees to join unions and be able to engage in Union activities to improve their working condition.
Under this Act, it is illegal to terminate, suspend or take any other adverse action against an employee for their union actions.
It is under this last paragraph that this question falls.
If Wal-Mart had acted in anyway adverse towards employees based on the Unresolved People Index (UPI) alone, then their actions would have been illegal. They did not, so they are operating in legal territory.