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.
Car bomb in Syria's Latakia kills at least seven (Reuters)
from the article: Alice Walker gets new invite to speak at University of Michigan