Personally I think this is probably just because at this time of year it kind of makes sense not to worry too much about Commercial Policy, flights are running extra full anyway. Agents at ARN, and indeed everywhere, appreciate people are anxious to get home or whereever, and basically did you a favour. Generally speaking this won't happen, so the advice above is definitely correct, you just were a bit lucky here. Otherwise this is not just against policy but the AA ticket tends to be an extra barrier to flexibility.