Why is an article "a" or "the" omitted before "duty-free"?
Native speaker’s answer
Rebecca
An article isn't used before "duty-free" because "duty-free" refers to all of the stores at an airport. "Duty-free" stores are stores that do not have import or sales tax. If he would have referred to one store and said, "duty-free store," then he would have used an article before "duty-free."