I must side with Delta on this one. If you go to the grocery store (in a state that charges say a 10% sales tax on groceries) and buy a coke for $1.00. I don't think that the store can waive the $.10. Why would they want to lower their price so that the tax is included?
I realize that it seems a bit absurd on the surface - but I imagine that it is a lot simpler to collect the $.10 instead of trying to find a way to adjust the fare basis in order to pay the appropriate tax to the government.
IMHO, I think that Delta deserves a break on this one.