Coinbase may argue it applies to all your purchases but I think it would be pretty hard for the feds to argue it applies to you using your crypto to buy pot in-state, especially considering how few dispensaries take crypto, and after all the portion of the commerce clause in question is called the "interstate" commerce clause for a reason. Affecting interstate commerce invokes their jurisdiction.
I'd be much more concerned about the dispensary that's actually taking the crypto. I wonder what POS system they are using that has crypto payment integration and integrates with their sales system.
For the record they don't force you to. They accept FIAT $USD as well. I think most people right now are pretty much doing the same thing aren't they? I mean buying BTC with $USD and then selling it and converting it back to $USD so it's not like they are HODLers