A BOD member, who is also a DA of Scruitneers, explained it to me. The Miata came to IT from Showroom Stock, where it was required to use the hardtop so retained it in IT. The old roadsters didn't come to IT from this direction so weren't allowed the top.
When he made us remove our hardtop,(perfectly by the rules) it slowed us 7 mph on straights and 3 seconds a lap. So, I suppose, allowing tops to be added would have to be considered a performance adjustment.