Recently (Oct 2016) I flew from New York JFK to Brussels. While at JFK I bought a lipstick at the duty free shop. The lady saw my boarding pass and said that since I was flying to Belgium I had to pay tax.
Anyone know why?
Duty and sales tax are two different items.
"Duty Free" means what you buy has not been charged duty for being imported into the USA (assuming it was made outside the country). But your purchase may still liable for local sales taxes.
"Tax & Duty Free" shopping is similar, but your purchases are not charged local sales tax.
One would guess the shop you visited was simply "duty free".