Would a judge approve the warrant though? Just because this person communicated with Assange?
As to your second sentence, I think it's very reductionist. I see a FISC warrant as functioning like a subpoena. Say the SEC subpoena's your company's financial records from your accountant, and finds in there evidence that is sufficient to get a proper warrant to get documents from your company's premises. Does that mean there is no protection or purpose to the SEC not just being able to give itself Article III warrants?