[Jeff Lee]

My understanding on how some are getting away with this is the factory had a coil spring and inside of that coil spring was a shock. The argument goes that the outside diameter of the coil spring is not defined so in essence, a smaller diameter coil spring while still having a shock through the middle of it is legal. Same principle as OEM but vastly different in appearance.
Of course you can shed a lot of weight (example: an AMC 1970 and newer has a very large outside diameter spring), it makes it easier and probably cheaper to find the proper spring rate for a drag car and of course the ride height is considerably easier to adjust.
I believe this trend started with the 4th generation GM F-bodies. The same ones that got rack & pinion steering...