Southwest Airlines will start charging some customers for checked bags, ending its unique free policy in an effort to boost earnings. The airline's "bags fly free" policy, allowing two free checked bags, will change on May 28. The move follows pressure to revamp its business model and increase operating margins. Southwest will continue to offer free checked bags to loyal customers with the highest status and those who pay premium fares. The changes come after activist investor Elliott Investment Management criticized the airline for not charging bag fees to boost revenue.