I hear it was a trick from the organizing company. Some airlines were conducting interviews, but not from walk-ins, they were interviewing previously screened applicants, whom were scheduled for the interview in atl weeks before. Advertising on-site interviews on the organizer's website was misleading, and a cheap move by the guys putting the fair together IMHO, unless it was the Airline that pulled a fast one on them (seriously doubt it)...

I strongly recommend you ask first if the airline is specifically conducting interviews on site to walk-ins, before you pay.