That seems weird.. If you roll up and a site is available, then it goes un-rented. I found a couple NF's that went internet reserve but both you could rent whenever you arrived. Their problem was no cell service at the CG.. worst was they blocked the bay-at-post.
Any clue why they stop it at noon, day of?
That is when the reservation window ends.
With state parks, you can stay one night in the honor system. You pay in an envelope in a lock box.
They were rumored to allow this for the sites, but so far...that has not been established.
Before, you had to request the permit 3 days in advance. Although, usually, if you called in or visited the office they could get you the permit.
Most forest offices are closed weekends and holidays. So, this does allow for reservation on days you could not have before.
It is give and take for sure, but this system is better overall for last minute adventures, if you make your decision by noon.
Ultimately, with every system, there are work arounds. My guess would be if someone was to occupy an unoccupied site and no one reserved it last minute, if you had $16.50 on hand, you would just be requested to pay it.
But, until they make amends for the honor system, it would be at the discretion of the ranger to cite or fine.