Hmmmm. Interesting. I was told by AA once (or twice?) that whether the taxes were waived depended on whether the majority of the ticket was paid with the tax exempt voucher, or not. If the majority (more than 50%) of the ticket was paid with a voucher, then the taxes were waived, whereas if the majority was paid with real money, then taxes would be charged. I don't know if they really knew what was going on, but that was what I was told.