OP is wrong and took advantage of the system by trying to bully.

1. He was not entitled to a fee-free change or a fee-free cancellation. DOT rules requiring a 24-hour fee-free cancellation only apply to bookings made at least 7 days in advance. OP says "immediately" and I take that to mean <7 days. B6 follows the DOT rule and chooses not to be more "liberal". That is clearly stated on its website.

2. The rule permits a fee-free cancellation. It does not permit a fee-free change. Sometimes that amounts to the same thing and a carrier will permit the change rather than a cancellation and a new ticket. That all depends on fare bucket availability. But, the rule is clear on this point.

"Debbie" may well get her hat handed to her by her supervisors if they listen to the call. But, it's way over the top to come here and blast B6 for following the law and then following its own policy.