The relevant statute, 18 U.S.C. § 48, criminalizes (a) "knowingly creat[ing], sell[ing], or possess[ing] a depiction of animal cruelty with the intention of placing that depiction in interstate or foreign commerce," though with an exception for (b) "any depiction that has serious religious, political, scientific, educational, journalistic, historical, or artistic value."
"[D]epiction of animal cruelty" is defined in (c) to include "any visual or auditory depiction ... of conduct in which a living animal is intentionally maimed, mutilated, tortured, wounded, or killed, if such conduct is illegal under Federal law or the law of the [jurisdiction] in which the creation, sale, or possession takes place, regardless of whether the maiming, mutilation, torture, wounding, or killing took place in the [jurisdiction]." This means that it's a federal crime to distribute videos of cockfighting or dogfighting in, say, California (assuming the depictions lack "serious ... value") even if the cockfighting or dogfighting was legal in the place (say, Puerto Rico or Japan) in which the video was created.
The statute was enacted as an attempt to stop the distribution of so-called "crush videos," which generally depict a woman's legs and feet, often in high heels, stepping on insects, mice, or kittens; and it does indeed seem to cover such videos, assuming the relevant state law bars the underlying conduct (often yes as to killing kittens, often no as to killing insects). Don't ask me why people would want to watch this stuff, but apparently some get their jollies this way.
But the statute is written much more broadly than that. On its face, the statute would also punish, depending on how judges and juries interpret "serious religious, political, scientific, educational, journalistic, historical, or artistic value" (emphasis added):
A TV program showing foreign bullfights, which might be legal in the country in which they're taken, but illegal in at least some states in which the program is shown.
A magazine with photographs of people illegally killing endangered species in a foreign country.
A magazine with photographs of people committing cruelty to animals, aimed at exposing and punishing such cruelty, so long as the magazine is sold on newsstands or by subscription (rather than given away).
One can certainly argue that all the above has such serious value, but at least as to the first item and maybe as to the others, some factfinders might conclude otherwise — the test is quite subjective, and some jurors or judges might well say "this bullfighting scene has no serious value; it's just aimed to shock, titillate, and get ratings."
Note also an important difference between this clause and the third prong of the obscenity test, from which the clause is borrowed: clause (b) doesn't say that the work has to be judged "taken as a whole." This means the "serious value" exemption under this law may well be a smaller safe harbor than the "serious value" exemption under obscenity law.
The statute doesn't fit within the existing obscenity or incitement exceptions. President Clinton's signing statement tried to cabin the statute by saying that the Justice Department should construe the law narrowly, limiting it to "wanton cruelty to animals designed to appeal to a prurient interest in sex"; that at least brings it closer to the obscenity exception, though not entirely within it. But the signing statement isn't part of the law, and is certainly not binding on later administrations.
The real question is whether the child pornography exception — the one exception that allows restriction of the distribution of speech because of the manner in which the speech was created — should be extended to cover the distribution of material the making of which involved harm to animals, rather than just harm to children. The argument would be that, as with child pornography,
production of cruelty videos can be done in secret, but the distribution has to be relatively public;
a ban on production will thus be very hard to enforce;
so long as there's money to be made in distributing cruelty videos, there'll always be someone willing to produce them; and thus,
to prevent the harm that takes place when the videos are made (injury to animals), one also needs to stop their distribution.
The argument against extending the child pornography exception would be:
The statute might end up suppressing a lot of valuable speech, such as the film of the bullfight and the like, and clause (b) is an inadequate safe harbor because it's much too vague.
The statute will in fact suppress more valuable speech than child pornography law does, because depictions of animal cruelty are more likely to be relevant to political debates or to legitimate art than depictions of sex (or of lewd exhibition of genitals) involving children.
The harm that the distribution of this speech causes — indirectly furthering animal cruelty — is much less severe than the harm of indirectly furthering sexual exploitation of children. (The legal system itself embodies such a judgment — child sexual abuse is a very serious crime, generally punished much more severely than animal cruelty. Cockfighting, in particular, is not even a crime in Puerto Rico, though Congress could have outlawed it if it wanted to. For more on when and whether it's legitimate for courts to draw such crime severity lines as a constitutional matter, see Crime Severity and Constitutional Line-Drawing, 90 Va. L. Rev. 1957 (2004).)
This also illustrates how the "slippery slope" can work in a legal system that's built on precedent and analogy. Crush video laws have indeed been advocated by their supporters as analogous to child pornography bans; and while courts might well draw the line between the two, perhaps on the grounds that child sexual abuse is just much more harmful than crush videos, the analogy seemed to be at least helpful in persuading legislatures to enact the laws.
Some might embrace the slippery slope here, if they think that cruelty videos should be banned. Some who disagree about cruelty videos nonetheless might accept the slippery slope risk, on the theory that child pornography is so harmful that we should have an exception for it even if there's some risk that the exception will spread further than one would like. (That's my view, and the Third Circuit decision suggests the risk of spread isn't that high, though note that the 3 dissenters did indeed rely heavily on Ferber as justification for carving out a new exception here.) But one shouldn't pretend that the slippery slope risk doesn't exist.