They decidedly went overboard in trying to cover them, though. Just as they went overboard in excluding hundreds of countries from participating in all but 1 tournament.
The site's Terms & Conditions could easily have said: "You must be at least 18 years of age to participate in tournaments on this site." (not "to use this site"; Ed is absolutely right that such a prohibition makes no sense)
(And they could have required international players to forfeit any prizes prior to entering any tournaments in which they're not yet eligible to win any prizes for legal reasons.)
I know, from participating in many randomly-drawn multi-player games, that many kids do use this site, enjoy playing the game, and have probably got their parents to buy some equipment for them in the ProShop. I don't think there's anything wrong with that at all; in fact, it increases WGT's popularity & revenues.