If I am driving down the road, and there is someone biking or walking and I don't see them and I hit & kill them, it is still an accident. You can bet your ass I'd be in jail for it.
Just because it happened on a racetrack doesn't make it much different at all. It didn't happen during the course of a car accident (I.e. Dale Sr.), it happened under caution, when cars are supposed to slow down and drivers are supposed to be even more attentive to the situation around them, especially right at the site of the accident, with the driver out of his car. On top of that, TS is arguably in the top few dozen professional racers/drivers in the world. He knows how cars work, how to handle them, how to proceed under caution, and to stay alert.
Intent aside, TS ran over killed a guy; accident or not, he needs to be held accountable for Kevin Ward's death.
That is not even remotely similar. If someone went strolling down the center of an interstate and you hit and killed them, there is zero chance you go to jail (well assuming you weren't drunk or high or driving on a suspended license).
This kid walked down into the center of traffic. More than 1 car had to swerve to just avoid Ward HIMSELF standing in the middle of the track. Remember his wrecked car was at the top of the track so it isn't like they could avoid him by going high either. Hell another car almost hit Ward while he was still up towards the top of the track (check out the video around the 30 second mark) And besides that, there is an inherent risk to being a race car driver. When you step out of your car on the track while the race is going on, that risks becomes much greater. When you walk down into on-coming traffic, the risk is multiplied exponentially.