The authorization bill was in effect until midnight last night, so any tickets purchased up until that point would have required the US excise taxes, etc. be charged. Beginning at 12:01, when the bill expired, there was no authorization for the airlines to collect these taxes on behalf of the FAA, so was no longer necessary on tickets beginning at that point.

Definitely no justification for a refund if you bought a ticket before then. Haven't checked AC, but what might happen is your US portion of taxes affected by the bill on an itinerary to/from the US are no longer collected by AC. UA is definitely not charging these taxes at the moment. US, on the other hand, while not charging these taxes anymore, did institute a fare hike to compensate, so that total fares (including taxes) basically didn't change, however, the portion that would have been collected on behalf of the US government/FAA went to themselves instead. A little disingenuous, I think, since they are likely not going to rescind these increases when the taxes come back (though they may rescind it based on the fact that other carriers don't seem to be following suit).