It's America's (crappy) solution to our relatively low number of vacation days.
This would suggest that you'd find less conference participation in places with more generous vacation policies, like Western Europe or Australia. Anecdotally I haven't seen any evidence that that is true, although it might be.
What I have heard from various people is that conference attendees in the US tend to be much more focused on socializing, whereas in Europe most people will attend the sessions and then go home. But shorter travel distances may also be a factor.
On a related note: alcohol laws may also make a big difference. Europeans have pretty much grown out of the "awesome, I can drink legally now" phase by the time they hit 25, Americans in that age group tend to have a slightly immature alcohol culture.
I think this is a major part of it. In Europe many kids are drinking like crazy when they're 15. By the time they're 21, binging and going nuts is sort of passe. Underage drinking is also not seen in the same way as it is in the US. The attitude is more "it's better not to drink when you're a kid" rather than "kids drinking is the root of all evil".
I think there is a much more mature view of alcohol consumption. On the other hand, many Europeans seem to have a much more hard-line view on drugs (even marijuana).
But when you consider the history of the US, it is not surprising. We are still less than 100 years away from Prohibition.
This would suggest that you'd find less conference participation in places with more generous vacation policies, like Western Europe or Australia. Anecdotally I haven't seen any evidence that that is true, although it might be.