In this case, probably justified. Hurricane Isaac took a big toll in the area, although not on the same scale as Katrina, and it will take a couple of weeks to clear the backlog of out of towners. Hurricanes on the Gulf Coast put a lot of pressure on many different types of businesses, especially anything hospitality oriented. You will see some price gouging in certain areas, but it's just how it works in a stressed supply and demand local economy.
Rereading your post, though, it would depend on how far in advance we are talking about, and what the refund policy is. I am a little leery of parks I don't know requiring full payment without a "right of refusal on arrival". In this case, I believe this park is well reviewed, and I might not have that same issue if I was about a week or so out.