That's usually done if the contents are criminal or infringe a trademark, etc, not just for any reason. Each TLD operator can essentially establish their own rules but they have to let ICANN review and if you have open domain sign-ups then ICANN will get mad at you if you break the rules you committed to (this will mostly be handled in private and there's not much chance a country will lose the right to maintain their own TLD, but private companies operating TLDs might)