It's a seemingly legitimate way for companies to give their employees a perk. Sure, some people are actually there to learn things for work purposes, but the vast majority are simply happy to have a few days off work to relax (and maybe learn a bit) on the company dime. It's America's (crappy) solution to our relatively low number of vacation days.
It shouldn't be any surprise that people drink and party a lot at conferences. People tend to do those things while on vacation.