For certain airlines/routes/destinations, there can be good deals. It may not seem so for some of us, who live in i.e., relatively low alcohol price destinations, but for others, it can be a very good price. Also, there's the convenience factor for those who really don't care about what things cost.

I would not be surprised if the DF vendors got fed up with all the attempted chargebacks/disputes by those who had their DF purchases confiscated at the connection point if they didn't get their liquids in checked luggage.