I can't make much sense of this guy's reasoning but it's fairly common in the travel trade to pair Friday & Saturday nights together, so that you can't book just one of those nights. (For example, the majority of B&B establishments in Brighton do this).
However there will be plenty of alternative campsites in Suffolk that don't have this policy. If you'd like some suggestions just ask. (I'm in Suffolk). If you do so, please state whether your definition of 'campsite' means static caravans, touring caravans or tents.
If you mean tents or touring caravans, you might like to take a look at this site. My friend often stays there, in her campervan, and I've been to visit her many times. The site is small enough to be quiet but big enough to have a bar. It's situated right on the cliff tops, looking out to sea. It's a great place for touring around the Suffolk coast:
http://www.cliffhousepark.co.uk/index.html
Chris