NO. Only PTO volunteers are allowed to "help out", and "space is limited." That's why I'm so ticked.
It sounds like the school is not all that interested in making money. It's much easier to get a parent to buy something if the parent sees the products to buy. Very, very odd situation. Parents have always been welcome to attend the book fairs at our children's schools. Before school, during school, after school. We must be fortunate to have such open schools.
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.