Technically not allowed to give the answer to that.
But, think of it this way. How is the furnace suppose to know if its running in HP heating mode, or cooling mode.
Well, the furnace is connected to the Y1 and Y2 terminals. The blower shouldn't care if Y1 or Y2 is in heating or cooling as long as the W1 and W2 terminals aren't receiving a signal. But, I guess having the O terminal connected does add another layer of protection to prevent the aux heat from coming on while the HP is in cool mode.