I can't honestly say that I understand why one uses "pour le" instead of "au". Perhaps it is just a set expression.
They mean different things:
On a pris l'avion pour le Japon = We took the plane for Japan
On a pris l'avion au Japon = We took the plane in Japan
Same as in english.
We took the plane for Japan.
Nous avons pris l'avion pour le Japon
We went to Japan
Nous sommes allés au Japon.
What about the other exercise where Duo insisted we just jusqu'à? Why isn't that right here?
Maybe it's also the plane "for" Japan as opposed to the plane "for" some other destination. Sometimes the French is more specific than English is what I'm thinking.
This sounds quite odd in English. Is it also odd in French? It implies there was only one plane. We would normally say something such as "That day we took a flight to Japan". I know that's not a direct translation. :-)