For those following the recent controversy at the University of Michigan surrounding the case of a professor who agreed to write and then declined, on narrow political grounds, to write a letter of recommendation for an undergraduate upon learning that the student wanted to pursue a semester abroad at a university in Israel. The professor expressed himself in an email to the student with great politeness. You can read this article about the American Association of University Professors AAUP position against academic boycotts, with a link to the full statement and an interesting reference to South Africa under Apartheid.