Do DOT rules require airlines to respond to follow-up statements, or only to the original complaint?
I don't believe they require a follow up statement however the DOT has what seems like a case administrator that looks over the complaints and responses. So if AA says "you weren't profitable to us so we cancelled your tickets" and you respond "that doesn't seem legal" it might be in their best interest to respond instead of letting the DOT purse enforcement and fines.