You have a 24 hour window for a deal like this and you know it costs you $10 for everyone who games the system. So you let the hackers think they've won then after the 24 hours is up, reveal that they've lost. Instead of finding creative and more difficult ways to game the system, the hackers wasted their 24 hours partying and getting drunk and so LivingSocial wins.
Probably not exactly how it went down, but it's a good story.
They clearly weren't validating all of the form inputs on the server side. Hopefully this was a learning experience for the engineering team.