I admit the line on this is very blurry, because if I were an asshole and wanted to fully stretch this line of thought then pokemon booster packs could be considered gambling, but this specific case isn't really defensible when it comes to the ethics violations.
A lot of schools banned D&D for the same reason, because they couldn't ban things just for having pentagrams. This may not be the same case, but Magic cards also were also called a 'gateway to the occult' while I was growing up.