It would seem that one practical effect of such a law is to protect consumers from inadvertent possession of child pornography, when buying or renting DVD's or video streaming from legitimate companies, at least for media produced in the United States, Generally, all main western democracies (the UK, Canada, Australia, France, etc) have similar laws. But off the cuff, there might seem to be a "risk" with material coming from (or originally produced in) non-western countries, or possibly Russia or some of the former Soviet republics or satellites, even if later distributed through American media companies. It's not clear that ages of actors would have been verified during production in less westernized countries.
Ironically, for all the furror of Russia's anti-gay propaganda law (said to mirror COPA in the theory behind it) and all of Putin's talk about protecting children, Russia doesn't criminalize mere posession of child pornography. But an American who possesses it in a country where it is legal could still be prosecuted under US law when he returns.