I'm not sure how burning wood is carbon neutral. Trees, other plants, etc. will sequester carbon from the atmosphere at the same rate regardless of the source. All that matters is how much gets into the atmosphere.
Because the trees sucked the carbon out of the air to make their wood as they grew, and if the trees are left to decay naturally, the carbon returns to the air.
It's just the basic carbon cycle: plants remove carbon from the air as they grow, and return it as they decay, (or burn in a natural fire.)
