If you are doing commercial photography, you need a pass, and maybe even a permit from the SCTA Media folks, listed in the Rule Book. As for being a spectator, I can't think of a motorsport event that allows the spectator more access. You have complete access to the pits if you're on foot. You have access to the sides of the starting lines of all the various courses. And back from the starting line, where the vehicles are lined up, you have complete access. Plus if you talk nice, almost every competitor looks kindly on your shooting his entry. Nothing to worry about, enjoy yourself.