That's exactly right. They are heuristic. Control charts were designed to be used on a manufacturing floor by workers who updated their charts manually on paper and called an engineer to aid in further investigation when a manufacturing process might to be out of control. Making out of spec parts or having an expensive machine break because it's operated outside it's limits cost real $$ so some false positives on a low frequency signal can be worth it. Also manufacturing engineers or QC would be responsible for the frequency of testing and sampling procedures so it's unlikely to be as naive as you might assume from the outside.