Merchandising is about finding the mix of products and labeling that connect with the customer. Like AtomicDog said, there is a significant overlap among people who are interested in real space and scifi. I also suspect that it is even more important among the typical shoppers for these kinds of models, mostly young people who were born well after the Apollo missions. The scifi category is a place that they are likely to stumble upon them. I noticed that the site also lists the LM and CSM models in the "other" category. But keep gently badgering them and maybe they will set up a real space category. It can't hurt.
