That's the semantic game. If they have authority to limit financial transactions by people, then they have authority to limit financial transactions by people facilitated by things. And things can not (at least not yet, not until general AI exists and recognized as independent legal entity) initiate transactions by itself - somebody caused them to do that, by setting off chain of events that resulted in that transaction. That person can be prosecuted if the transaction is illegal.
> If Iran secretly plants a cherry tree in Central Park, is OFAC allowed to sanction the cherry tree
What you mean by "sanction the cherry tree"? If people use this tree to, say, exchange money (OFAC does not regulate eating, AFAIK) - for example, by hanging sacks with money on its branches - then yes, OFAC would be able to prohibit this way of transferring money to Iran as well as any other way of doing it. Why not?
> They are arguing no, because OFAC has no authority to sanction trees.
It has authority to sanction financial transactions. The means by which they are executed is secondary - it's be stupid for Congress to give them authority to regulate transactions made in one way, but ignore same transactions with same effect made in slightly different way. I don't think anybody would have any luck convincing the courts that was the lawmakers' intent.