As the previous posters have stated, TK has the right to sell tickets on some of their "tag on" intl-intl segments, but not all of them. That determines if tickets are for sale or not.

If they are for sale, the price is largely determined by the airline's need to sell tickets on these short segments. If most of the traffic is going through to/from IST, they are not interested it just selling the short segment in the local market.

Every FNA-OUA seat they sell is a FNA-IST seat they didn't sell - complex revenue management tools will dictate what the best balance is.