I'm not 100% sure why other than that the whole point of duty free is to avoid paying local taxes. I can speak from experience: I once tried to buy a bottle of liquor and a giant bar of Toblerone at a duty free (at LAX TBIT) before a domestic flight. I was firmly told that I couldn't buy the alcohol, but they still happily sold me the comically sized Toblerone (with local sales tax added.) My guess is there must be some legal limitation preventing duty free stores from selling liquor to domestic passengers, even if they charged the proper tax.