What does the manufacturer or the test pilot that conducted the initial testing say??
I'm unable to access that information - I was assessing the aeroplane,which has been around for over a decade for other purposes, and this was just an interesting peculiarity that I picked up whilst learning my way into the aircraft before flying it.
My suspicion is that the US manufacturer found the spinning characteristics compliant with part 23 but a bit unconventional - certainly the POH uses a non-standard spin recovery. That is probably at the root of the "no spinning" requirement.
Why the UK didn't allow aerobatics again I don't know - that wasn't part of my assessment, but the lack of spinning permissions does seem quite feasible. Personally I think that the F4 comparison isn't particularly valid as the height loss in a spin in a fast jet is so great that Martin-Baker rather than Parkes is more likely to be the solution to an inadvertent spin, and partly because piston engined aeroplanes put a big torque generator in the nose which perhaps make a spin more likely from a mishandled aerobatic?
(The other possibility of course is that the UK engineers overseeing certification weren't happy with the structural reports at the higher g limits, which wouldn't be a first. Opinions do differ between regulatory regimes about acceptable practices in that regard as well.)
G