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?
I'll cover money laundering first since it seems to get so often repeated here:
No, exchanging bitcoin for cash, or vice versa, is not in and of itself money laundering. Replace "bitcoin" with "casino chips". Is every casino in Vegas behind bars? No. Why not? Because there was no intent to launder, only an intent to exchange value for value.
Secondly, the concern over bitcoin dealing being a Money Services Business (MSB) is a little more legitimate. IIRC, FinCEN has already stated that these sorts of businesses are MSBs, and require all the usual compliance measures attached to that label. There are also some states that consider bitcoin trading businesses MSBs as well, on the state level. On a practical level though, hardly anyone is enforcing these rules. Does that mean there's no risk? No, of course not; there's definitely a risk. It's just not currently that large of a risk.
Lastly, with regards to unreported income, that's an issue with any income, regardless of source. If a stranger handed you $10 out of nowhere, that's income and you're required to report it. Hell, if you sold some crack to that stranger in exchange for those $10, you're required to report it. Unreported income is not a bitcoin specific concern.
Your reaction is a bit overblown. People have been and are currently buying and selling fairly large amounts of BTC both in person and online on localbitcoins alone.
Is this going to be an ongoing thing? I actually just "semi-moved" (couch surfing with friends) to SF last week and I'd be interested in trying this out but I still need to update my Github quite a bit...
I dunno about you but I've never tried working naked in a jacuzzi after having finished an entire bottle of McCallan 12 Year so I can't say I wouldn't enjoy it... maybe he just knows something the rest of us don't.