they belong to but still...
Non-practicing or occasional Mass-attending Catholic voters tend to vote based more on the larger group within which they live and work: Their voting patterns reflect their education, economic class, region in which they live, etc.Why blacks vote overwhelmingly liberal is a question that may be less interesting than the mere fact that they do in unison. Whatever their shared experiences, education, brotherhood of blackness, grievances against Republicans, etc... they certainly share a common outlook. We are asked to "think with the Church" and blacks certainly think with each other politically, although I'm not sure their's is necessarily a good model. I suspect part of the uniformity is coercive in nature in that if you don't think with the group you'll get ostracized, ala Clarence Thomas and other conservative blacks.
"Shunning" certainly works as a social force given its success in the Amish community where 85-95% stay in their communities. I don't know that it would work too well with Catholics today, given the general lukewarmness. Shaming is mostly outside the mainstream nowadays. A birth out of wedlock used to be a badge of shame but now is almost a badge of honor where the alternative - to kill the child - is so routine. To shame or shun seems like "tough love" in a society averse to that sort of love. It's perhaps self-perpetuating: by the Golden Rule we do until others as we would have them do unto us, and we would not like to be shunned.
Still, I do wonder about the long-term viability of any society where "shame" is defined so downwardly as to be nearly invisible.