I think this change has been in the works for a while and the decision didn't surprise me at all. With the "robust" version of the board it was obvious they wanted people to engage in Facebook style "streams" or conversations with likes and everything instead of posts. I'm sure they want to use some of Facebook's features and traffic analysis engine in their new conversations format and this requirement will let them do that.
I don't like it but at least in theory that should stop the PatsLoses and BostonGirls from annihilating a conversation...