Cancelation fees should be massive, it deprives use of the land if there's not enough time to find a new user. Our national parks are a limited resource, don't reserve and take away use from someone else if youre not going to use it.
To put it another way, they could pay a $40 fee or lose $100 by doing nothing. Most people would rather pay the fee and get their $60 back.
Not surprisingly, this is also the exact model that hotels use...