What do you do if someone moves into the campsite across the way while you're camping--can you pull out with them there? And, if so, how is it that you cannot back in with them there? (And, if not, isn't it slightly hypocritical to expect them to move temporarily so you can leave, but not expect that you need to move temporarily when someone who has a reservation for the site you're using shows up?)
I can, at least to some extent, understand the desire to have some first-come, first-serve, nonreservable sites. However, given that they've done away with them, it really seems that the best way to ensure you don't have to move in the middle of your stay is to make a reservation--especially as that's really the main purpose of having a reservation, guaranteeing you a (particular) site.
While there's no harm in asking the party who's reserved your site if they'd be willing to move, I don't think it's the least bit unreasonable for them to expect that they will get the specific site they reserved, even if there are plenty of other open sites. I know I sometimes have particular reasons for reserving a particular site (rather than just choosing one that fits more or less at random).