Americans love tradition and there is no greater tradition than the idea that if you can't be fired, you will do better work.

Okay, that is actually the antithesis of America, but in the late 1700s the US government made it possible to create a special class of employee - a tenured member of academia that would be free from cultural pressure, namely religious.

http://blogs.hbr.org/2013/03/its-time-for-tenure-to-lose-te/

Obviously, cultural pressure still exists today: Good luck finding Republicans and minorities with tenure. You simply don't get tenure until you survive an academic, cultural or political litmus test. It has become the very problem it was created to prevent.