The point is that users who come here to read or participate about the stated topic of a stock-specific board should not have to waste their valuable time weeding through a litany of off-topic noise. Why would anyone think they have earned a right to inflict their noisy banter on others?
There is no possible justification you can make for allowing off-topic posts on such boards because every one of them dilutes the quality of the dialog.
That said, good moderators know how to maintain a flow of dialog that allows for some off-topic content while not allowing it to drown out the signal or be unduly burdensome on those who are there for the on-topic discussion.