Part of the issue is "where" you can do this. If within T-30, you need to be present, at the gate, not someplace outside of the airport. For pretty much the same reasons you need to check-in by T-30. If the process is done online or using a phone, you could be "anywhere." I see no problems limiting this to the gate and honestly, GA are a bit busy at T-30 to handle a new ticket -- so limiting it to changing an existing ticket is very reasonable in my mind.