I don't necessarily think that Jack HAD to become a smoke monster. There's a critical difference between his experience and the MiB - the light.
When Jacob kicked the MiB into the cave, the golden light was in full effect. Actually it was probably the brightest we ever saw it. Maybe that's relevant, maybe not. Jack only went into the cave after Desmond (who's apparently immune from the effects of whatever electromagnetic energy is present) pulled the plug. That apparently stopped whatever mechanism created the smoke monster since the light was distinctly red until Jack replugged the hole. The only question, for me at least (and only this sole topic
), is whether or not the smoke monster-creating mechanism is in proximity to the pool jack sat in. Willing suspension of disbelief lets me assume that it was far enough away not to change him.
Then again, maybe that's what Hurley and Ben protected the island from post-show.
