They make street bike and car racing and kart specific helmets. In karting people use any of these depending on what they prefer. You see a lot of street bike helmets being used in karting
Arai makes a karting specific helmet called the SK-6. Essentially a non-Nomex version of their open-wheel GP-6 helmet. SA-rated car helmets and K-rated karting helmets are pretty much the norm for karting. I have an SK-6 and love it. I also have a Sparco WTX-7 Air Carbon which is my current helmet that I like a lot.