We have to admit, Kal is pretty plain in what he's trying to do here: Light people on fire. He even admits freely that burning is a hilarious game to him, and that he really enjoys people's fear of death. Nobody can blame Kal for misleading us about exactly what his horrifying machines do. Indeed, it seems as if he's actually trying to warn you away. Instead, blame the art crowd for interpreting these statements as the main thesis points of a revolutionary performance piece and not, more accurately, as a sociopath's description of a "flamethrower prison for the innocent."
The Fire shower is built to a set specification. If you aren't grown to the exact proportions it was intended to hold, there are no safety precautions. If you're too fat, hey, no problem! It'll burn those unsightly pounds right off of you in a frighteningly literal fashion.
That may seem like a joke, but seriously, burns are not actually that uncommon. People frequently emerge from the shower "missing hair and smoking." Although to be fair, the Fire Shower does rely solely on volunteers--you have to actually walk up to it and step inside, even after it's explained to you that it is a walk-in barbecue for fat people. It's not exactly going to sneak into your home and replace your normal bathing station with its desperate, fiery embrace. Well, not that's been proven anyway.