I too wondered why just showing you International ticket at the register would not suffice in a mixed-use terminal. They do that in Singapore Df (which is odd because every flight out of Changi is Int'l , lol)
Local liquor store sell .75th, 1 liter, and 1.5 liter sizes for most of the common brands, so it is not a "free 33%" in duty free here. Ounce for ounce, the duty free might be a couple of dollars cheaper. You could in theory do the refundable Int'l fare, and then sneak back out and enjoy your complicated scheme to save 5%, but keep in mind that you had to endure TSA to get to that point.
A better example would be cigs. Those are half the street price (in Newark, anyway), so it would depend on how many cartons you could buy at a time. Possible cash maker, but you won't get rich from it. I imagine the airlines would notice habitual ticket cancellations and raise a flag.