Yes, it is illegal, no arguing that. I also understand the safety concerns. It does make me wonder, how would anyone even know if it has been refilled? There is no outward signs that it has been refilled, I can have 2 cans next to each other, one refilled and one new/used, there would be no way to tell. So, unless someone watched you fill one, then watched you put it in your car, and saw you drive away with it, how would anyone know? Then, what evidence would be used to convict you? Without a video, how could it even be proved?
I am not saying this to encourage anyone to break the law. I just have to wonder about the law and how to possibly enforce it. The age of a tank does not mean anything either, I have old tanks still full, and new tanks that are full. Neither are refilled, both new.
Again, I am not advocating the breaking of the law or doing anything unsafe, please don't do it. Just thinking that this would be a really hard law to enforce.