I think you're mistaking volunteering for a different task as them not being paid - these employees are salaried, they're being paid for this. It's basically just doing 2 hours of warehouse work instead of 2 hours of their usual corporate job. Potentially some people would have to do additional work after hours to make up for work they didn't do in those two hours, but I assume busier people wouldn't volunteer.
I think you're mistaking volunteering for a different task as them not being paid - these employees are salaried, they're being paid for this. It's basically just doing 2 hours of warehouse work instead of 2 hours of their usual corporate job. Potentially some people would have to do additional work after hours to make up for work they didn't do in those two hours, but I assume busier people wouldn't volunteer.