Here's an example of a real hastle.
A French friend was flying from Paris to Tahiti. He had a layover in Los Angeles. Because the US makes EVERYONE go through customs and passport control, regardless if they just have a layover, he had to obtain his bag, go through customs and passport control, then recheck the bag.
Technically, he was not entering the US and his final destination was non-US.
If I flew NY - Frankfurt - somewhere in Africa, I wouldn't be required to touch my checked bag, or show my passport in Frankfurt.