David,
By no means am I an expert, but the dynamics of a crash on the human body in a very light-weight kart with no harness or roll cage are entirely different than what you would see in a 2000-3000lb car with a full cage and harness.
Just as there are differences in helmet requirements between an auto racer (Snell 'SA' rating) and a motorcycle racer (Snell 'M' rating) I suspect that the governing body(s) for karting have their own requirements.
I've heard about the issues regarding the horse collars from my Hans rep as well, but I think the above photo says it all.
2002 Cen-Div ITC Champ
(Converted to G-Prod in 2003)
(Bumped to H-Prod in 2008)
2008, 2011 HP Cen-Div Champ
2011 HP National Champ
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