I can't speak from any experience, so this is just conjecture. It sounds like a question that really should be asked of someone at the National Park Service rather than here if you want an accurate answer of what is permitted/expected.
I'd imagine that there would be a little bit of latitude for hosts (or others they report to) to do this sort of thing in at least a few circumstances. For instance, they may have had to move a camper from their reserved site because that site became unusable and needed repair or maintenance. Or perhaps there was some sort of a screw-up in the reservation system that somehow resulted in a double-booking.
