Just read this whole thing, missed my shot at an answer. Agree with LBrain. It is not like LMAD. The prison cell was chosen for him. So was the Kings decision.
You could look at it this way. Before the guard told him C, there was also a 2/3 chance A or C was chosen too, exactly like B or C. Eliminating C, which was beyond their control, still leaves A or B at 2/3, which then becomes 50/50.
Has to do with the wording. If A had gotten to choose which cell to be in before the guard told him, then the guard told him, the Monty Python thing applies. Just my take. I'm sure they'll be a lot of debate on this.