That's one way of tuning the S30 chassis. Here's my opinion on where that setup came from and why:
The 240Z, even heavily modified, is a car that does not turn in well compared to other race cars of its day (RX2/3, 911, 914, etc.). Add to that the fact that most CP racers of the 1970s (and most ITS racers of today) ran a welded rear diff which induces a lot of understeer on trailing throttle (corner entry) and you have a car that doesn't want to rotate into a corner.
To counter that a lot of the CP and ITS racers ran (run) more roll stiffness in the rear to loosen up the back. That additonal roll stiffness combined with a little bit of trail braking gets the car to turn in reasonably well and keeps it neutral mid corner (keeps the understeer away). With a welded diff corner exit speed is not affected by inside rear wheel lift or a little bit of slide as long as the driver can balance the car with the throttle.
Its a setup that's worked well for decades given the technology available during the 1970s, 1980s, and early 1990s. It still works well today on tighter, bumpier tracks
With a greater avialablity of clutch pack, Torsen, and Quiafe LSDs and more sopisticated shocks, some racers started experimenting with reducing rear roll siffness (smaller rear bar and springs, bigger front bar and springs). This can allow earlier throttle application in a corner and faster corner exit speeds. It took some devleopment time for those setups to start working but by the early 2000 it became the fast tune for ITS 240Zs. Chet Whittle proved that with a couple wins at the ARRC running a clutch pack LSD, Shock Tec double adjustable shocks, and a 200hp Sunbelt L24.