You're comparing apples to oranges. Ancient and prehistoric wildfires--some set by native hunters--may have happened more frequently, but they happened when the human population North America was probably less than 5% of what it is now.

Those fires, also, were  among the major sources of greenhouse gasses from natural sources. Now, human contributions to the carbon cycle--in part from industrial logging and agriculture--have thrown off the balance in which ancient wildfires were normal.  There are fewer acres of intact, biodiverse forest and grassland--only about 5% of old growth forest remains--to absorb and store greenhouse gasses. 

Your claim about ancient fires is akin to what climate change deniers say about ancient climates: that those climates were much hotter and harsher than today's and that "the planet was just fine".  This is true, but what it leaves out is human civilization. Human civilization will not be "just fine" if the climate regime changes radically