1. Unless you have time, pay the fee and then seek a refund. Make certain to include your e-ticket receipt showing the segment allowance as you believe it to be.
2. If the refund does not appear, file a DOT complaint. As the destination is the US, US law applies. Do not mention IATA. It is a trade organization for air carriers and has zero impact on US legal decisions.
This is simply incompetent on the pat of AS. The DOT rule is simple and the reason for requiring that e-tickets specify the baggage allowance is to avoid complexities between carriers and having to train agents in local variation of rules. Moreover, all it took was for the AS agent to ask a supervisor or her own help desk and she would have had an answer/
There is something to be said for filing a DOT complaint even if AS refunds the fee as DOT does keep track of the metrics of complaints and will take enforcement action if AS routinely violates US law. But, life is also short and perhaps not worth your time.