Fires caused by sparks from engines are categorized as "equipment." I did a search and couldn't find any breakdown of what types of equipment are involved.
The causes of fires by rank varies across the country with lightning topping the list in the West. Human caused fires of various sorts falls below lightning, which are arson, incendiary (cigarettes, debris burning, etc), campfires. Fires started by equipment follow. This includes fires started by hot catalytic converters and sparks because some people that aren't too bright and feel spark arrestors aren't needed.
We have areas here where ATV aren't allowed because there have been a number of fires started by them because someone decided to riding through dry grass with their ATV on a 104º day in July, which anyone with an ounce of sense would question the possibility of starting a fire, again, because some people just aren't too bright.
It is unlikely that the ATVs themselves are the problem, but the behavior of some of the people that ride them. It is the same principle that gets forest trails closed because a certain number of people insist upon taking their 4X4's where they shouldn't.
Smokey the Bear isn't an environentalist's mascot. Most of the environmentalists believe in letting things burn. Burn all day all night, burn everything in its path including houses, businesses and hospitals. It is more likely that someone that hated ATVs slipped that ad in as a campaign for their cause.
Control burns are done throughout the country, especially in the West. However, there is a huge difference between keeping up with control burns in South Jersy and keeping up with them in the Sierra Nevada, which is much larger than South Jersy. It just simply can't be done, so they do control burns around towns and along high-risk areas. Control burns DO NOT prevent fires, but they do make it easier to control fires in those areas if a fire ever breaks out.
A lot of you are in the Midwest or the East. You just simply cannot compare wildfires there with wildfires in the West. There is a big difference of having several days of dry weather before it gets humid again. Having consistently dry weather, low humidity, and having gusty winds bring in lower humidity a few days later, which is what we have been having in Northern California, which is why there are non-stop public service announcements about fire danger in the summer here.