It's not just Olympics that capitalize on volunteers. Any industry that is viewed as glorious/glamorous with a potential of high payoff (not just monetary, but also in terms of fame and reputation) exploits volunteers and low-paid workers to death.

Acting ("When someone in LA tells me 'I am an aspiring actor', I ask 'Which restaurant?'"), fashion, arts, music, charity, design and architecture to some extent. Take for example http://workinculture.ca/ You would never see that many volunteer and unpaid internship positions in IT or engineering.