Being as I heard Obama speak at a Black university, and did not notice any obvious change in the way he spoke, I imagine that you just heard it somewhere and have been repeating it ever since. He used the term "brothers" to refer to other Black people, that is all I noticed.
It is not illegal or even deceptive to change one's accent when talking to people who speak the same way. Actors do this all the time. It is simply a talent that some have and others do not.