Forest fires are not net contributors, because the carbon that is returned to the atmosphere by combustion of the biomass was removed from the atmosphere earlier by the process of photosynthesis in the plants.
By the same logic, you could say that oil, coal, and natural gas are not net contributors either. They all originated from plant and animal life from earlier times.
Guess it gets back to the old law of the conservation of matter. Whatever changes there are, there is no new matter on the earth, only different forms of it.
That's not great logic, because the carbon in plant biomass was recently removed from the atmosphere (you could stretch this back a few hundred years for the longest lived plants), while fossil fuels go back millions of years, and that carbon was effectively removed from the Earth system until human activities "reactivated" it.