It is illegal b/c of the course of action kids may take. Kind of why 2nd graders aren't allowed to watch the Sopranos. B/c at that age and really until their mid to late teens, kids are very impressionable and outside influences are a big deal for them. And we don't want our country to become like Russia's and have mail order brides available for delivery from the age of 13 on.
The reason is society and what could happen to the children of the country. If a girl sees all her guy friends watching porn she'll want to do what is in the video (not complaining) to make them want to look at her and for her to be a bigger part of the group. This happens anyway, but would be more common if not illegal for under 18.
So yea, porn is a no no for kids under 18 b/c they don't have the power to make decisions involving their life b/c of outside influences and peer pressure.