It's been like that for ages and It's all pretty clear on their website:
Helmet Standards approved for Race for Real Off-Street Meetings
Drivers Drivers of vehicles slower than 10.99-seconds (1/4 mile) are permitted to use any helmet (full-face or open- face) meeting one of the following Standards:
• SNELL SA2000, SA2005, SFI 31.2A (USA)
• SNELL M2000, M2005, SFI 41.2A (USA)
• BS 6658-1985
• AS/NZS 1698:2006 or AS/NZS 1698.