Well, that's all true, but I think this is one of these cases where we should care more about context of justification issues than context of discovery issues. After all, the causal story about why I'm a liberal isn't very interesting (liberal parents, liberal teachers, etc.) the issue is what kinds of reasons can I offer in defense of my view.
Short answer, and a genuine question: why do you think we should care more about justification than discovery here?
Why you, MY, are a liberal might be a standard boring tale, but why many people aren't, or why many people, when making ad hoc decisions, decide not to be, seems important.
And if what we'd consider soundly grounded reasons aren't what convince people to adopt one view or another, that also seems important.
I agree with you, but I would hope that what might start as 'a bric-a-brac of ad hoc likes, dislikes, etc' would be subject to the same critical review that is attributed to 'systematic thinkers'. The only real moral position that might not receive this treatment would be that stemming from a decree, say, a passage in the Bible. "Thou shalt not steal." Clear, from God. Right?
So I'd agree that liberals are less likely to make descisions that control others' behavior, stemming from their recognition that each person has the right to his/her opinion, and that opinion is backed my some sort of critical thought.
Well obviously the causal reasons behind people's political positions are important in general, but I don't see at all how they're important for this particular conversation. That is, how would the causal story have anything to do with refuting Brooks claim that the liberal "autonomy about end of life decisions" position is morally thin and relativistic?