How likely is it that the CG will fill the spot you all of a sudden decide not to occupy? Not likely, so they are loosing revenue. Forfeiture of your deposit gives them a little cushion to the blow.
I don't own a business, but I can see their point of view. Clicking the "I understand" button before being able to confirm the reservation on a website without actually having read and understood the policy has its consequences.
I would have taken the gracious offer to change the reservation and took my chances. Hope your back mends.