He shouldn’t take such a stupid pledge which is contrived to put him on the spot because it immediately demonstrates slavery to the media. He shouldn’t play their game. His refusal is entirely consistent.
People look at the surface and think they are seeing the “workings” of negotiation. Most have not been in a high stakes negotiation and were successful. You give away NOTHING unless it is less important to you than what you get back. What would Trump get back? Full commitment to him as the leading candidate? So if he would sign that pledge...what does he get in return? NOTHING! This is why Obama and Kerry should not negotiate the Iran deal. They get a signed piece of paper for Obama’s “legacy” and Iran gets the bomb. Hell of a deal, huh? (Amateurs!)