I guess I can see it from the producers' standpoint. I'm sure those performances cost a ton of money, and there are a million things that could go wrong, including an instrument either dropping out or being out of tune or something. I understand them wanting to ensure that doesn't happen.
I don't agree with it at all, but I understand. Besides, it's not like the Super Bowl halftime show is like the pinnacle of artistry, anyway, let's be honest, it's pretty much the biggest sell-out thing a musician can do.