Had it been a purely internal communication, it would have quickly leaked to the press. The press would then distort and misconstrue the info to make Basecamp look as controversial as possible. Better to get in front of that by having a polished public communication.
They knew they had employees who would attack them for the change either way. Being proactive seems pragmatic. It's also on brand for them, unlike a lot of other companies where it would seem strange for the founders to suddenly post a controversial blog post. Not commenting on the merits of the change, just how they communicated it.