I'm sure someone else with more knowledge than I will chime in but let me take a shot. It's been my experience when dealing with the kiosk for an earlier flight that, yes it will offer you the earlier flight for $25. What the kiosk is actually asking you is "Hey! Do you want to guarantee yourself a seat on the earlier flight? If so, click here!". If you want to fly stand by for free, don't take the $25 option. Just get your original boarding pass and go to the gate or talk to a ticket agent in person. If you want to fly early for free, you need to head to the gate and wait til the last second for your boarding pass. Now, if the flight is wide open the GA is likely to hand you a boarding pass early but I don't think you're going to get a kiosk to hand you a boarding pass for an earlier flight without the $25.
Reasoning is that right up to the last minute passenger B may be willing to cough up the dough to get the guaranteed seat you are waiting to get for free.
My experience only, YMMV.