Think about it this way: You're a liberal, everything in your social life is a performance of progressive piety. With your family, with your friends, with your coworkers, etc. You've spent years cultivating the image of a Good Person. Then something happens to you or someone you care about, something perpetrated by a brown savage. All eyes are on you now to make sure you're staying true to the faith. "Yes, I'm still all for open immigration, but..." is a sign of heresy. You'll be excommunicated on the spot. And where would you go then? Become friends with right-wingers? YUCK!

Think about it this way: You're a liberal, everything in your social life is a performance of progressive piety. With your family, with your friends, with your coworkers, etc. You've spent years cultivating the image of a Good Person. Then something happens to you or someone you care about, something perpetrated by a brown savage. All eyes are on you now to make sure you're staying true to the faith. "Yes, I'm still all for open immigration, but..." is a sign of heresy.