Restate the problem thusly, and it all become clear.

When you are presented with three doors, you choose one which you do not wish Monty to discard. It doesn't matter what's behind it. You're not trying to pick the prize door.

Monty opens a door, which has no prize behind it. You are now presented with a simple choice - and here's what you know:

One door has the car, one door doesn't. That statement is 100% absolutely certain.

NOW, you make the choice - which is it? Door A or Door B? It's a 50:50 choice...