It isn't against the law necessarily, as obscenity laws in the US only make it illegal if the medium is an image depicting a child and there is no artistic purpose (very vague). Regardless, It annoys me when organizations do this when they can easily just create a ToS outlining what isn't allowed. It truly boggles my mind the contempt that some orgs have at creating and maintaining a ToS instead resorting to hiding behind fantasy laws as justification. Just straight face tell your users "Nothing related to X of any kind or permaban", very clear, very succint.