I'm neither a lawyer nor familiar with US law/regulatory stuff or US consumer - but I assume the point is that it's not legal to sell it as a fire extinguisher, nor to use it to satisfy fire extinguishing equipment requirements for your office building or whatever. Not that you're breaking the law if you have in your home, whatever you want to use it for (unless you're required to have a fire extinguisher, in which case you'd just have it in addition, if I'm correct).