Well, I'm not sure that that's how you should look at it. If it's 'on top of everything else she has to do', then it's also on top of everything everyone ELSE has to do, too! It's a required component of the program so regardless of the credit status, kids have to do it and obviously, if it's required, then it's a do-able thing.
I also don't think you should conclude that they're 'implicityly saying that dance isn't important to them'. Again, I think you really need to discuss this dance/credit issue directly with someone at Northwestern. It's not a fruitful discussion for all of us to speculate as to why they do it this way.