You have a President who is denying that the Russians influenced the election, an FBI investigation that the NSA may or may not be obstructing and a Senate committee investigation that may lead to impeachment of a sitting president. Because of this leak we now know that the President is deliberately lying.
How does withholding that information not harm the people?
For the same reason as the US leaking the information on the UK terrorists wasn't good: you may disturb and even destroy an investigation in progress. If details lead suspects to destroy evidence or take a "diplomatic" trip home you've not exactly done the country a favour. As far as I'm aware, that investigation is still ongoing.
Biting the hand that feeds IT
