Their excluded from sales taxes which go up to 9.55% state wide and higher in some locations. Taxing everything else but X, is a subsidy on X.

Only international fuel is duty-free for obvious reasons. Domestic fuel is taxed, though the states (IL, GA) with large hubs have curiously low taxes on kerosene.

An exemption when everyone else is taxed on the same commodity is a subsidy - the untaxed people/companies are free-riding on the rest of the infrastructure/services/etc. for which the other taxpayers are paying.

The result of giving a $100 tax exemption/credit is literally the same as no tax exemption/credit and sending a $100 check to the subsidized entity (except for the reduced overhead & bookkeeping on both ends).

Absolutely it is. If everyone else is paying, but not you, you're being subsidized.