"It's not fair" -- Not only are they unwilling to cope with that truth but they perpetuate it through their elitism and hypocrisy. Their lack of maturity is more about a bad conscience: on one hand supporting equality as socialism and on the other their impulse to raise themselves above it, hence the term Limousine Liberal. They buy into the American dream but are at odds with it. But odds are they they know this and are happy supporting a cause that makes them hypocrites. Why? Because they are rich hypocrites which is better than being poor hypocrites... if Gods asks they can always say, "We're all fallen... and I tried, honestly... my heart was in the right place."
I think a lot of the more extreme liberalism - being terminally offended, for example, or knee-jerk reactions to things as a result of racism, sexism, etc., is really am extreme form of immaturity.