There are the duties that have to be paid, but airlines also have to apply for state-specific liquor licenses as well in order to serve alcohol on the ground while the door is open. So in addition to paying the duties, they have to pay hundreds to thousands of dollars annually to maintain each state-specific license. This is perhaps why there is inconsistency route to route (prices fluctuate per state) and why even they may fluctuate on the same route (maybe the price has increased, maybe the license hasn't been renewed, etc.). I still think it's pretty cheap of them to not do it, but just wanted to provide some additional context since it isn't just the duties that would prevent them from doing it.