I'd be willing to bet one, or both, of the following:
1)that they have ex-hippie parents
2) that they are merely parroting an anti-American version of history as indoctrinated by their teachers.
That having been said, I used to hang out with people a bit like that, in my younger, stupider, liberal days.
It was a childish phase. I got over it.
Just like liberalism in general.
Can I add one because I was like these kids? Something isn't right at home and in an attempt to get attention they sh*t all over their parents, usually conservative, values in an effort to gain attention. Speaking unfortunately from experience.