Walmart doesn't need nearly as many employees as it has, not even after the layoffs. The government needs companies to create jobs so people aren't in need of social assistance, so they give incentives to companies like Walmart to provide jobs, far more than they would otherwise.
In all likelihood, your job isn't necessary at Walmart, they only have you so you don't become an anchor on the economy. You'd be surprise how much work someone can do when they suddenly realize they may be unemployed if they don't start working hard.
When those laid off are gone, the rest will have an epiphany of insight, that being their job is not as secure as they thought it was and they had better start working as if their mortgage and car payment depended on it.
Because it does.