Because, in the overall context of the discussion, the taxes are a red herring. They don't change the outcome.
The comparison between DAL and HOU, while not perfect, is probably pretty close. The two metropolitan areas are of comparable size in the same state. They both have a smaller, older airport dominated by WN, and a larger, newer airport dominated by a legacy carrier.
The only other city that can feasibly used as a comparison would be Chicago. The Metroplex is far more similar to Houston than it is to Chicago.
Here's an interesting article that speculates that AA's recent fare reduction between DFW and Nashville is an attempt to placate the Tennessee legislators that are trying to repeal the Wright Amendment or at least extend the Amendment to cover Tennessee. AA claims it's a coincidence but I don't buy it. It looks like a good old-fashined bare knuckles brawl and will be fun to watch.