Depictions of animal cruelty are not, as a class, categorically unprotected by the First Amendment. [...] Since its enactment, the First Amendment has permitted restrictions on a few historic categories of speech—including obscenity, defamation, fraud, incitement, and speech integral to criminal conduct—that “have never been thought to raise any Constitutional problem,” Chaplinsky v. New Hampshire, 315 U. S. 568, 572. Depictions of animal cruelty should not be added to that list. While the prohibition of animal cruelty has a long history in American law, there is no evidence of a similar tradition prohibiting depictions of such cruelty.1 [boldface added]The First Amendment itself does not even hint at "permitted restrictions.” However, legal tradition has allowed restrictions in the categories listed above. The unfortunate effect of this situation is that citizens can't take the Constitution at face value.
Categories of unprotected speech aren't automatically made illegal. It's just that states can pass laws restricting speech in these categories without the Supreme Court raising a fuss. Or it would work like that if the categories were more clear. Obscenity, for example, has been defined by different tests which are themselves unclear.
The Hicklin Test
From ~1870 to 1957, the definition of obscenity in the United States was often based on a British formulation:
[A]nd I think the test of obscenity is this, whether the tendency of the matter charged as obscenity is to deprave and corrupt those whose minds are open to such immoral influences, and into whose hands a publication of this sort may fall. Now, with regard to this work, it is quite certain that it would suggest to the minds of the young of either sex, or even to persons of more advanced years, thoughts of a most impure and libidinous character.2The Roth Test
In 1957, the Supreme Court declared the Hicklin test unconstitutional:
The Hicklin test, judging obscenity by the effect of isolated passages upon the most susceptible persons, might well encompass material legitimately treating with sex, and so it must be rejected as unconstitutionally restrictive of the freedoms of speech and press.3...and recognized a new standard which some courts had already been applying:
whether to the average person, applying contemporary community standards, the dominant theme of the material taken as a whole appeals to prurient interest.4One major difference here is that the average person rather than “the most susceptible persons” is considered. It also matters whether the offending material occurs throughout a work or just in some parts. And the standard is allowed to change to fit “contemporary community standards” regarding speech about sex.
All of this makes the Roth Test problematic. Who is this theoretical “average person”? How large is the community under consideration? And do we count the community of the writer/speaker, the reader/listener, or anywhere the work is advertised or sold? At what point do elements in a work count as “the dominant theme"?
The Miller Test
In an attempt to “formulate standards more concrete than those in the past,” the 1973 Supreme Court established the test which is currently in use.
We acknowledge, however, the inherent dangers of undertaking to regulate any form of expression. State statutes designed to regulate obscene materials must be carefully limited. [...] As a result, we now confine the permissible scope of such regulation to works which depict or describe sexual conduct. That conduct must be specifically defined by the applicable state law, as written or authoritatively construed. A state offense must also be limited to works which, taken as a whole, appeal to the prurient interest in sex, which portray sexual conduct in a patently offensive way, and which, taken as a whole, do not have serious literary, artistic, political, or scientific value.5On the plus side, the requirement of specific definition in state law – depending on how state laws are written – might actually provide citizens with some idea of what is legal, without waiting to find out how some judge feels about it.
The problem of vague standards persist, however, because the question of whether a work appeals to “the prurient interest in sex” is still dependent on what a hypothetical “average person, applying contemporary community standards" would decide (see context of the quote above). What counts as a "patently offensive" portrayal? And what does it mean for a work to have “serious” value in one of the areas mentioned?
If you couldn't tell already, I'm not a fan of subjective laws. A citizen should be able to read the laws to find out ahead of time whether a given action or expression can be legally condemned. As Justice Douglas wrote in his dissent:
Obscenity - which even we cannot define with precision - is a hodge-podge. To send men to jail for violating standards they cannot understand, construe, and apply is a monstrous thing to do in a Nation dedicated to fair trials and due process.6
1. United States v. Stevens, 559 U.S. ___ (2010).
2. Regina v. Hicklin (1868). [full text]
3. Roth v. United States, 354 U.S. 476 (1957). [full text]
5. Miller v. California, 413 U.S. 15 (1973). [full text]