That's due to the inherent smugness of liberalism. It's "default-ism". Liberals think that everyone is like them, because that's the most sensible and logical position. That's also the real reason they are a liberal. It was presented to them as the default position when they were kids, so they latched onto it, because they didn't really want to spend time or effort exploring the other positions (oh look at that, propaganda aimed at kids). And because humans project, they assume everybody is like that.

So they'll proudly say things like "I'll proudly die for your right to say things I disagree with". Because in their heads, there's no way different ideologies will grow or prevail in a democratic society. They treat you like a peculiarity, an amusing curio, a nuisance at worst. Then the Nazis start taking advantage of a crisis, and they start growing in numbers, and pretty soon they are electing representatives and presidents, and suddenly the liberals will start going into rants about how the Nazis ought to be investigated, so they can be locked up and silenced.