I am not a big fan of motorized vehicles, I generally bike. But it seems to me that saying "stunt riding is fine as long as it doesn't hurt people" is more or less the same as saying "drunk driving is fine as long as it doesn't hurt people". That is if we are talking about stunt driving on a public road (haven't read the whole discussion). stunt driving is much riskier than calm driving and when you do it in a public road you transfer that risk to other people, including pedestrians and non motorised bikers, who don't want to be involved.