He could have waited, but he felt you violated the rules and did what he’s supposed to: dismiss you. Humiliating? Perhaps. But it’s not like you were singled out for undeserved punishment. If this was your 4th time at the SAT, you should have known that anything resembling notes would have raised serious questions.
Treat this as a lesson learned, and move on. Life’s too short to stress over mistakes made. You will be fine.