If a company is conducting a mass layoff they have to give at least 60 days notice to employees affected, unless it's a liquidation or other urgent situation. Also, some states require that the state be notified of the mass layoff. Giving people severance is so they don't sue the company; as part of severance the employee usually signs a document waiving their rights to sue. Severance is not required by law in most states.

You can go to this website to see which employers in Georgia are conducting layoffs that require notice: WARN Website Data Listings - Georgia Department of Economic Development

I don't see Expressjet anywhere on there, which probably means that as of now there is no announced plan for widespread layoffs or shutdown of the business. The people who are being terminated are probably part of a smaller number of layoffs that doesn't trigger the thresholds for Federal WARN Act reporting.