There is no marketplace for copyrighted content and there can't really ever be one unless you mandate compulsory licensing. If you want to watch Frozen then you have to buy from Disney or one of their approved vendors. You can't take your business elsewhere and so from a monopoly standpoint it doesn't matter who actually owns it. You might not like that Amazon in particular owns MGM because because it means that their content will likely only be available on Prime but nothing was stopping them from making an exclusive deal prior to the merger.