You're thinking of arguments in the conventional, everyday sense. The legal system deals in legal arguments.
The point ctdonath was making was that the Supreme Court has ruled that private, personal cultivation of marijuana, even when in accordance with state laws, has an effect on interstate commerce. The relevance of the effect on interstate commerce is that the activity - growing marijuana - falls under the purview of federal law and federal law enforcement agencies.
To bring it back to your suggestions and the distinction between conventional and legal arguments, ask yourself this: for each suggested argument, think of two worlds with one being a world where your argument is absolutely correct and the other being a world where your argument is absolutely incorrect. Is there a difference between these two worlds in whether or how personal marijuana cultivation affects the illegal interstate marijuana market?
Following your line of arguing (based on your characterization of the SC's logic), all activities can likely be shown to fall under the purview of federal law and federal law enforcement agencies as essentially all activities affect your local environment, and in turn all activities that interact with your local environment, which will include many activities directly or indirectly impacting commerce which will then impact interstate commerce.
This line of argument can be used to justify federal involvement in anything. This makes it useless in drawing any interesting distinction, nor does it address any of my original arguments.
I do think there is some truth to the argument that individuals' actions have global consequences.
However, I do not think the US Supreme Court has real claim to be the final, global arbiter. Usurping power is, however, the US Supreme Court's demonstrated prerogative. It's pretty much how the institution began its life.
The point ctdonath was making was that the Supreme Court has ruled that private, personal cultivation of marijuana, even when in accordance with state laws, has an effect on interstate commerce. The relevance of the effect on interstate commerce is that the activity - growing marijuana - falls under the purview of federal law and federal law enforcement agencies.
To bring it back to your suggestions and the distinction between conventional and legal arguments, ask yourself this: for each suggested argument, think of two worlds with one being a world where your argument is absolutely correct and the other being a world where your argument is absolutely incorrect. Is there a difference between these two worlds in whether or how personal marijuana cultivation affects the illegal interstate marijuana market?