Import Duty
  
No, it's not an obligation to import. (Although we do need the oil...so, um, Saudis, thanks a tonne...and here's a few billion a month to pay for it.)
No. An import duty is just an import tax. Like...for every barrel of oil we import at our cost of, say, 40 bucks, we'll slap on a duty of 5 bucks.
Why? Well, we need the 5 bucks for Congressmen's mistresses' hotels. They're not cheap. (The hotels.) We also want to "protect" a bit our own U.S. domestic producers of oil. Like oil found near Mark Cuban's house in Dallas. So we make it a bit more expensive for producers of cars in Japan, and of wine in France, and of those little stackable Russian dolls...to sell them into the U.S. versus what these things cost in Japan, France, and Russia.
The U.S. has a modest Protectionist policy relative to the rest of the world. Import duties are kind of a trade weapon, and those weapons are scary. A few countries (think: Korea in telecom) only make a few things...well. And the U.S. is a huge buyer of, say, Samsung phones. If we tax the crap out of those phones to favor Apple, our own most-loved technological fruit, then Korea could be seriously economically damaged. So we try to be thoughtful about what taxes we are levying, where, and how.
Ugly things can happen when we're amiss. Even uglier than those stackable dolls.