That really is an outrage. I've never heard of parents not being allowed at the book fair. But you know what? Lots of kids won't be buying a darn thing. And many others will only be buying when their parents accompany them in the evening.
Go ahead and ask if your son can stay back, and offer to stay with him in the classroom, but I wouldn't make a big stink unless you think your son won't be staying in the school. It never helps a child if he feels different in a way that makes him sad.
I'm not planning on making a stink. You know the old saying about honey and vinegar? I will be my most rational, calm, and diplomatic self. If I get nowhere, it's ok. At least I tried when no one else who shares my sentiments would.