I've got a slightly different take. It's a judgement thing.
Nobody likes to be judged, told they're wrong, or that their actions are wrong. If I'm not held to a common moral standard, I can do anything I want, no matter how perverted I know society thinks it is. By extension, if I don't want people to judge me, I need to protect the rights of those on the extreme fringe, because admitting that anyone is outside the boundaries of morals, ethics and good taste would put my "rights" in jeopardy.
As a result, liberalism takes the concept of non-judgementalism to the extreme, and fails to judge ANYONE on ANYTHING--unless it's you, judging them. So, when folks with a strong moral compass say things like "he's a criminal," "homosexuality is a sin," "NAMBLA is a bunch of sick, disgusting individuals," well, that's wrong, because who are you to judge them. Of course, the hypocricy of them judging you so harshly escapes them. The end result is moral relativism that demands you to accept that the only truth is there is no truth. Figure that one out.
Since you can't blame or judge individuals, but you need a reason for why things happen, the rational result (if you can call it that) is to either blame someone faceless who isn't really responsible (i.e., the bar who sold the drunk the last drink before he took out a minivan), inanimate objects (guns) or a very select group of "safe" individuals who by their ideological nature threaten their ideals (Christians, conservatives, Big Oil, etc.)
Remember, it's all about emotion. I get angry when someone judges me, first because I feel bad that they may be right, second because I don't like to be humiliated, and my self-esteem suffers when you tell me I'm wrong to do something. Therefore, judgement (also called "interolance") of any sort is bad, wrong, and to be stamped out wherever it's found.
The end result is a world where statements like "We will not tolerate intolerance!" make sense to people.