Why does the politically correct environment do their best to stifle free speech when it's not their own?

Could it be that the ultra-left have a most difficult time defending their positions with logic? Therefore, before placing themselves in that awkward position of challenging well-thought-out points of view, they attempt to demonize or outright dismiss opposing points of view.

In a recent controversy that arose over the commencement speaker (Johns Hopkins Hospital Director of  Pediatric Neurosurgery, Dr. Ben Carson) at Emory University in Atlanta, biology professors attempted to cast doubt on the legitimacy of the choice of an individual who was a creationist and went so far as to accuse the individual of saying that evolutionists were unethical. They came to this conclusion because the speaker feels that is very easy to explain the source of morality if one believes in biblical principles.

However, if one believes in survival of the fittest it becomes more difficult. In the survival-of-the-fittest model, killing someone or stealing from them to enhance your own position is natural. Therefore it is legitimate to ask the source, how does one derive morality in such a model or how does one even define morality?

Rather than demonizing someone who asks that question, wouldn't it be more reasonable to actually come up with a logical answer, if one actually exists? It would seem that the purpose of an institution of higher education would be to explore such questions in an open forum, rather than return to the suppression tactics that characterize the Dark Ages.