Its not easy knowing ahead of time, especially if you cant drive or often have erratic work schedules. The purpose of a tenative is to show your interest in the event.
I myself, have something similar to a ~90% chance of going to a game after saying tenative, most of the time it leads to confirmed.
And btw, one of the previous comments made a metaphor to leaving an item on hold in a store. A confirmed status is this, not a tenative. I dont mind if someone confirmed before me and takes my spot. Thats fair. So Im not preventing anyone else from going.