One way tickets are a red flag for the inane TSA computer. "Hmmmm... one way ticket? Must be a terrorist!" Then, once you're flagged for the extra special treatment from security, you're SOL with online check in. I know US and AirTran are the same way - not sure if its everybody. I typically fly RT, but occassionally I'll book a one way and 99.9% of the time, it's a TSA interrogation for me.