It seems to me it's more a matter of suggested "guidance", and very likely a legal requirement to announce a rating, than something that is strictly enforced.
It was a good question for Riley to ask.
It's somewhat complicated if you exam the legal minutia of it all in the US.
In terms of literature, at the present time to my knowledge essentially information itself is protected under the First Amendment. Of course possession as well as the dissemination of
anything deemed legally classified remains a crime punishable by federal law.
However if you shift the media to
films and video it all changes, relative to statutory laws depending on the state in question. As to whether or not such a minor infraction like a "Class A misdemeanor" is strictly enforced, I doubt it. Though I suspect some jurisdictions may remain "dedicated" to rigidly enforcing much of anything related to perceived or imagined "obscenities" for political purposes. Particularly local district attorneys in election cycles.
The important thing has already been emphasized in earlier posts.
That the exhibitor/publisher is held responsible. Not the person merely viewing such materials based on established guidelines or outright statute law.
Example:
(Tennessee) Title 39 - Criminal Offenses
Chapter 17 - Offenses Against Public Health, Safety and Welfare
Part 9 - Obscenity
§ 39-17-907 - Restrictions on showings.
Universal Citation:
TN Code § 39-17-907 (2014)
(a) It is unlawful for any person to exhibit for public consumption, whether or not the exhibition is for compensation, any motion picture, film, movie, or videotape that depicts sexual conduct as defined in § 39-17-901, unless the exhibition is within a theater auditorium or other enclosed area that effectively removes the exhibition from the view of members of the public who are not voluntarily engaged in viewing the motion picture, film, movie, or videotape.
(b) No minor under eighteen (18) years of age may be admitted to a movie theatre if the movie has been found to be "harmful to minors" pursuant to § 39-17-901. It is a deceptive practice under title 47, chapter 18, part 1, to advertise or promote a motion picture as having a rating other than the rating that has been assigned to it.
(c) A violation of this section is a Class A misdemeanor.
Interesting to note that even this law in its past form only a year earlier was deemed unconstitutional and subsequently revised.