As usual, human intervention in ecosystems results in a disruption of processes that every organism in that system specifically evolved to live in. There are second and third order effects of allowing endless deadwood accumulation, pretty much guaranteeing that fires will be catastrophic. Not only do they cause super hot fires when it finally happens, but deadwood cover doesn't allow the usual plant life to take hold, increasing erosion and water retention, speeding the drying out of the forest.

Deadwood around lakes and streams can really screw up fish and other wildlife, too.

Well, it is not like the Americas were in some kind of pristine state of nature when European colonists arrived. The indigenous people of the Americas made lots of changes to the landscape, including the use of fire.

But it was a fairly consistent state of nature; as far as I know, those were old practices, and the ecosystem had adapted to them.