Yeah, that's pretty common. If you get a large amount of traffic, they take the domain from you and try to sell it back to you as as "premium" domain at a hugel

Yeah, that's pretty common. If you get a large amount of traffic, they take the domain from you and try to sell it back to you as as "premium" domain at a hugely inflated price. You don't have any rights to free domains. Freenom are the 'official' owner of the domain and can do whatever they want with it.