I share your viewpoint, as well. I've been researching this since last night and it's apparently very common for drivers to exit their vehicle and walk out on the track after crashing, especially once the yellow flag comes out. It also appears that one of the rules when they are under yellow is that they are not allowed to accelerate. Further, two of the eyewitnesses who were situated near that turn and stated they saw everything - at least one of which was a fellow racer - stated that there's no way Stewart didn't see him, and that while Tony likely never meant to intentionally harm the kid, he clearly gunned his engine and attepted to scare him, and in this case things went terribly wrong and Ward ended up killed. The driver (Graves is his last name, I believe) also said that he feels Stewart should end up in jail.
I watched the video before I knew he had passed and before I knew any of the details from witnesses, and the first thing I tweeted was how he clearly accelerated once the kid was near his vehicle.