I don't believe it is? While it is certainly common in airports I don't believe it's an airport-only purchase, it's just less common to find Toblerone in some parts of the world but most major confectionary retailers/supermarkets in Europe at least have Toblerone. You will find it in most stores that carry Mondelez International products. Much like Perfetti van Melle (who own among other things the Mentos Brand and Chupa Chups) products some brands may be more common in their native markets and frequently sold at airports worldwide but they are by no means exclusive to them. From an economist POW it is likely just the result of marketing and brand strategy. The result of a combination of a product frequently given as a gift (chocolate, alcohol, souvenirs etc), a fairly non-perishable product and a conscious effort by the company to brand and distribute their product to gift shops.