In America, universities offer professors tenure so that freedom of speech can be protected. I may not agree with Alice Walker, but she is exercising her freedom of speech when she promotes boycotts, which are a peaceful means of political influence. If you don't want violence, then you need to allow people to have other peaceful means of expressing their views.
Turkish foreign minister: Syria deal 'important step' to resolving crisis (Reuters)
from the article: Alice Walker gets new invite to speak at University of Michigan