As I've said before here, I want my DD to know that she can be whatever she wants without society deciding, assuming or even dictating that she should be anything because she's a girl.
For me, that's what this is about.
And if she wants to be a princess, will you allow her to do that?
Just picked up DD from CM and have been told it's superhero day for children in need 'but the girls can just go as princesses'.
It doesn't sound like they're saying girls MUST go as princesses... just giving another option. If it was dress up as a princess day or as a fairy day and they said children or boys can just go as pirates if they want to, would you get upset over that as well?
Perhaps a few girls that went through before you specifically stated that they didn't want to be a superhero.. or specifically asked if they could dress up as a princess instead... is it REALLY so offensive?? 
Sorry, I've got a daughter, and I just don't see the issue. I'd simply say "these are the options, what do you want to choose?" and leave it at that.