Variable Star question

Does anyone understand how the economy works in Variable Star? I am confused that in a universe without faster-than-light travel, the performance of the stock market on earth could impact the colony ship after it has left the solar system. I am also confused as to why shares would continue to be traded in the colony ship after it has left earth. After all, it will be well over an earth-century before the ship might be expected to return and I don't recall that there was even an expectation that the ship would return one day.