There are several factors in their decision making. I will say that the nt01 isn't necessarily a faster tire than ps4s. It can be more consistent, but for a hot lap (and especially considering a season of driving with variable weather) I think those groupings are fairly accurate. My best street tire time was on the yoko a052. Nothing 100tw+ can touch that tire for a hot lap, but after 3 laps it's grease city and after 12 sessions they're trashed.
The upper group is intended to be slower than the bottom group. What's insane is you can run a stage 2 gti in the tt5 group if you run a tire from the top group. Running a tire from the bottom group my stage 2 gti has passed tt5 and some TT4 cars pretty quickly, but it certainly doesn't come close to keeping up with a tt3 car. With a ps4s or one of the "old" 100tw from the top group it should be very competitive in tt5, as long as you have the dsg. Gearing and low speed corners favor dsg a good amount... And there's an updated modifier to account for it as well (because the tcr cars were trashing everyone in high spec cars).
A big factor is that this is club racing and not pro. While some people are certainly capable of plonking down $1k for tires each weekend to get the fastest times out, most people are money conscious. The groupings partially reflect this to account for those who will run one or two sets for a season rather than 6-10 sets.
Everyone has been unhappy with these rule changes, which means they're most likely fair!