NASCAR requires it.
The sanction only requires it at the touring series level. Of those it's not all 38.1 devices that are allowed. For example the Defender is not allowed. The local series (Whelen All American) where the majority of NASCAR licenees race does not require a head and neck. That is more of an apt comparison as it's basically the equivalent of club racing though it's for profit. Some of the individual tracks require it but it's not a sanction mandate. It's the same in with IMCA. Though most of the racers in the faster classes wear them.
It's a good idea though I'm not particularly pleased with the HANs Sport Model. If the sanction is mandating it stop with the insurance/lawyers/burden to society because those can't backed up as fact. Only opinion. The sanction is well within it's rights to say "look, that's how it is, your wearing one". At least the choice is any 38.1 device.