Requiring Snell approved helmets for autocross is pretty funny.
Nomex lining? If death or injury from fire is relevant in Autocross, then why not require driving suits?
And the Snell SA impact test is based on impact with a roll cage. Fair enough--if your car has a roll cage. Not in stock class...
Given the stock seatbelts many autocrossers use, a full-face requirement would make a lot more safety sense than making people pay a fortune for a pointless sticker.