I think there's a bit of a big assumption going on here about what was happening in America before the modern era.

It was quite common for native Americans to manage forests for game hunting, particularly clearing between the trees to allow for better chasing and shooting (arrows). Often they even did this with fire when it wasn't so dry as to set the trees alight, just the bush and debry. So, yeah, before any humans came to the Americas there would have been centuries where those fires raged by themselves but probably not for the last 15000 years or so.