I wouldn't say unprofessional, because this isn't a normal employer-employee relationship. Nate Silver is a famous professional, he made a business deal with a large media corporation, he made some money; later that large media corporation used the IP he sold them in arguably-bad ways, and he's upset. I don't blame him for being upset, certainly I don't think he owes Disney anything, but at the same time he's the one who agreed to sell his IP to a corporation and this is the kind of things corporations do with IP.