at sprint races on a yellow, cars are constantly pumping the accelerator checking grip and throttle response, it would be really hard to know which car was giving it a shot with many on a short track, and the camera was a fair way from the incident.
visibility isn't great in those cars with the roof wing and the roof board coming pretty low on the outside of the car, nobody is expecting to see somebody on the track in that (or any ) location. I don't think he meant to hit him.
In NY state there would need to be some evidence that would be hard to prove in court of it being intentional for a murder 2 or even manslaughter charge, but its the US so a civil suit will be coming, and the damage to Tony Stewart Industries, his career and team would lead to a quick settlement.
As a point of interest in the late 40's a midget car driver in NY was convicted of manslaughter and served time for trying to put a 4' car though a 2' gap and the result was the death of another competitor. It can happen.