First of all, Michelle Obama said on CNN that Sunday mornings are Family Time when she and Barack and their daughters would do things that were family-oriented.  Therefore, I am assuming that they didn't really go to church all that often.

Secondly, whatever was said at Trinity, remember, Barack Obama had never attended any other church other than Trinity, and had lived and been raised in a white culture.  Therefore, he would not have known whether this was part of what black people did in church or not.  Barack knew that it would behoove him in his Community Organizer role to attend a church.  I'm sure that he, being an outsider, found it very important to be accepted by other blacks, to be a part of a group of other blacks, especially since he was raised in a white culture.  Perhaps he even had a need to prove himself.

Third, for many blacks, the church is cultural tradition rather than a spiritual experience.  Most likely, it's that way for many other people groups as well.  There was a time, immediately following slavery, during the reconstruction period, when blacks who had extra money would join different clubs and fraternities.  This provided an opportunity to dress in their finery and be a part of polite society.  Many blacks did not have the initiation fee and/or dues paid to a club or fraternity, and their only social outlet was church.  Some blacks participated in both the clubs and organizations  and attended church.  Keep in mind, blacks didn't have country clubs and the like to socialize and get their respects.  So, church, for many blacks, was part of their cultural experience, something that was done on Sunday, an opportunity to dress up and have Sunday Dinner afterwards.

Fourth, Barack didn't experience Christianity until he was exposed to Trinity.  Therefore, he didn't have prior knowledge of what to expect. There are many people who have attended church for years, have sat in the pews, participated in the rituals, and still don't really realize what the Bible says or what it means.  A great many people, white and black, hispanic, whatever, have no idea what being a Christian is really all about.  The majority don't really have an indepth understanding of the Bible and the context in which it is written. There is no reason Barack should have been an exception to this. 

I am sure, by now, however, Barack has learned what a minister of God should not do.