Shortage of container slots for US export makes US officials to issue stern warning to liner companies about their obligation to carry US export cargos:
The idea is to fill the ships with as many empty containers as possible with destination Asia. This to restore the balance of the container shortage there. And so there is almost no room for loaded export containers on those ships. You sell your merchandise but then you cannot export it, also since transportation by air is often far too expensive at 3 euros per kg.