We have been attempting to relax our safety requirements, but still keep drivers safe. Newer cars have better restraint systems, crash protection, air bags, etc. Thus we have relaxed the harness rules.
Roll bars are another issue. I personally have seen 4 rollovers at track events. None were open cars fortunately. 3 of the 4 were beginners who made a driving error. The drivers were bruised up and a few scratches, but were okay. The 4th was a mechanical problem in turn 8 at Willow, which could have been really nasty, but the driver was skilled and damage was minimal. This was all in the days when we required 5 point harnesses. Have not seen a rollover with a factory 3 point belt, and I really don't want to!
The tire rules are there because street tires will keep beginners out of trouble. If you are into sticky tires, improved handling, more horsepower, etc, you really should invest $100-200 in a 5 point harness set up.
AROSC Competition Director