Exactly, I can't speak for every corps, but I've been involved in the past in auditions for a drum corps where hundreds are cut. When cutting these kids, they're encouraged to march somewhere else. I would say no more than 5% of those kids would even entertain the notion of going to a open class or lower tier world class corps. Everyone has their goals on where they want to be. Not marching in that dream corps and staying home to make money is the usual decision.
These extra 15 spots won't hurt the smaller corps. If your drum corps isn't good enough to attract enough people, then the solution is to get better, market yourself better, so that people will actually want to march there.