Standard operating procedure at many bracket races. Being old school, I don't really like them. It's like a fee for extra time trials. My home track (Cherokee Dragway) actually has TWO rounds of buy backs. Some tracks have one round of buy backs, some have 1st OR 2nd round BB (buy back ONCE 1st or 2nd rd). I prefer a "mulligan card" race (3rd round seems to be my toughest). You buy a "mulligan card" at the gate when you enter and use it to get back in the race ONCE any round except final rd. It can make for a long race.
Brian