I can't think of any good reason not to let people standby for earlier flights same day for free. If there is space in the system, why not move the pax right then? who knows what could happen later - what if there's a machanical and you have to overnight them? What if another airline cancels and wants to buy lots of seats on you? Better to use the open space earlier in the day, leaving mroe capacity for later when it might be sold.

Silly, silly. Just nickel and diming here and there.