I'm going to play devil's advocate for a min. While I agree with you regarding canceling ADRs, there is another side:
You have to call the ACTUAL restaurant to let them know you aren't coming if it's just a short time before your ADR. You can't just call disney dining (people all over the country) or do it through central reservations- the message doesn't get to the restaurant.
A situation I'm going to encounter- I really want to eat at Beaches and Cream. DBF really wants to eat at Cape May. I made an ADR for Cape May but we are going to try to get into Beaches and Cream first. If we aren't going to have to wait long for a table, we will stay, otherwise we'll go to our Cape May ADR and call Cape May to let them know we aren't coming. Is my plan unfair? Am I "taking" someone else's spot at Cape May by being unsure?
And by placing a CC on my reservation, would I lose money calling Cape May 30min prior to my ADR? How far in advance would a cancellation call be needed to not be charged? Actually I think that if a CC was required for ADRs, I wouldn't make any and eat offsite or CS. And if Disney has 6-9 open tables during typical meal hours, they need a new statistician. They aren't booking enough ADRs for that restaurant to account for cancellations!
PS You went during
Free Dining- I consider dining to be atypical of what you find during the rest of the year