Well, the guy who designed it (André Waterkeyn) apparently has copyright on every picture published. His family members and the Atomium foundation really do go after people who are using these pictures. I think it's ridiculous, but they must have the law by their side, since they're successful in getting people to pay those fines.
Here's another informative article about this:
https://www.unizo.be/advies/kan-u-een-foto-van-het-atomium-ander-bekend-gebouw-gebruiken-voor-commerci%C3%ABle-doelen