Wow! They let you bill them for time spent attending a party?
Well, it's on the honor system, so I guess it's my decision when I add up my hours to invoice. But I think it seems reasonable. The parties are explicitly organized as team-bonding events. The free food and drinks and interesting venues are nice, but I can't say I'd necessarily attend them if I didn't work there. It's similar to having dinner with a potential client.
Besides, it's a global Fortune 500 corporation; as a contract worker I don't get health insurance, PTO or any other benefits, so I figure they can pay me for a couple of hours of partying now and then.
Justifying, much?
Recently the boss sent out a message saying she'd be hosting a happy hour that very evening to introduce someone from another office. I asked a coworker if she was planning to go. "I don't think we have a choice," she said. (That seemed a little extreme, but still.)