I used to think that the seat was rollover protection too but after having them out I cannot see how it will do anything, the weight is in the base and the seat frame itself is not heavy enough to be rollover hoop unless you gently rolled over a field of cotton perhaps...And you need to be very short for it to help..
I know early Z never had the hoops and maybe it was a story to make people feel better?!
I have roll hoops and aftermarket seats al declare- insurance was no issue if using a proper insurer (not a compare the stuffed animal type place!)
Mmmm maybe having lowered suspension and wider wheels should be an insurance reduction for reducing the risk of rollover