I disagree. Even though you are flying the same day, you are still changing your reservation. They *could* charge you the $75 fee but they give you a break at $50.
You may look at it as filling an otherwise empty seat but the airlines look at it as you changing your reservation and now leaving an empty seat on your original flight. It takes their counter rep time and effort to make the change on both flights (new and original) and they charge you for it.
Besides, with flights being as full as they are these days, I don't really think this one will be much of an issue in the overall scheme of load management.