>>Those ARE the rules. PERIOD. FULL STOP.

Actually, no, you got them (subtly) wrong. After you have made your initial choice, you do not pick between two doors. You choose to stick or switch. Yes, the point is slightly semantic, but this seems to be where you're going wrong. It is *because* you can stick to the door you originally chose that the probabilities are not equal - because the probability you get a car to start with is 1/3.