The taxes pretty clearly have to be refunded. There is no statute or law that says the government can tax on intent to travel. Every single tax requires an actual act to incur the tax - in this case the tax wouldn't be due unless the person actually consumed the travel. That would be like sales tax being collected when a purchase is returned.

I tend to think that the carriers and the government collude on this using various flimsy excuses and regulations to keep from refunding the money. I also think the carrier imposed fees would probably have to be refunded as well as the taxes (the FAA still has some influence and fees are different than actual airfare).

Until there is a court case on this I do not think it will be settled. This would take a class action suit though to obtain the refund. It is simply a case of too little return for anyone but a greedy lawyer to take up.