There is one thing to charge for confirmed change in advance, another for standby at the gate with no checked luggage.

In the past, GA's wanted to get you on an earlier flight if there were empty seats. This opened up seats for later flights, which may be needed if someone missed their connection.

Plus some flights have notorious delays - like DCA-EWR in the afternoon. Missed connections cost CO money - when they have to accommodate you on another carrier.

I would think it is in CO interest to let people standby at the gate for free, when they have no checked luggage.