Why is there a piece of plexigl (or gl) in front of the Beach Boys drummer? Video included?

I was watching this video of the Beach Boys playing "Kokomo" and I noticed there was like a plexigl or gl window or wall in front of the drummer? If it's bulletproof, then give a story of why it's there. 10 points for answer wtih both the answer and story. a href="http://youtube.com/watch?v=1aN1CBa7S9M&feature=related" rel="nofollow"http://youtube.com/watch?v=1aN1CBa7S9M&f…/a