To SDcampowneroperator, one does not need to stay in the same spot for 14 days to incur 30 days of the additional $6/day. We could move around and stay in different areas, so your comment of the 14-day max is irrelevant.
You do point out that there is an annual use-fee pass for $18. I can assume, then, that if I have that pass, I could stay all year, and only pay the camping fees in various parks, and not pay the additional use-fee?
I agree with Monkey44, tell me the entire fee to camp, not just part of the fee. Campgrounds are not alone in this misinformation tactic, it happens all over. Tell me the all-in price, including tax, when I ask how much it costs to stay somewhere.