Why Use Por in Translating JFK's Famous Saying?

There are perhaps no more confusing word pairs for Spanish students than the prepositions por and para. One of our readers recently asked this question: In President John F. Kennedy's inaugural address, in the statement "Ask not what your country can do for you; ask what you can do for your country," why is the "for" translated by por instead of para? Our answer is here. If you can give an explanation makes the concept clearer, feel free to do so by commenting below.