Because of their feelings. No one said anything when Obama did the same thing.
Today everyone is affected by politics. There're no more intermediary layers because of social media and direct access to information.
Well, I hope all the people that have voted for Clinton (I assume those protesters mainly have) are not as emotive as the once mainstream medias are delightfully showing on their "news"... Because if it is, good luck trying to have a constructive debate, it is nearly impossible like that. I'm maybe naive to think there isn't as much division between the American people but those who are protesting are giving a very poor image of themselves, not because they are protesting but