(And if it wasn't "required", don't kid yourself---of course it was required.)
/endrant
After reading your post, I'm having a hard time seeing how this is the case. Perhaps people are not directly forced into doing weekend "hackathons", but I'd imagine there is quite a bit of indirect pressure to show up for these things for fear of not being seen as a team player. Although, maybe my own personal experiences have just made me too cynical.
I personally love my craft and often throw myself into it over the weekends, and outside of standard working hours. Since I also care a lot about what my company is working on (I am an employee/fellow, not a founder), that can include job-related work. Nobody at the company is asking me to do it, and I'm not doing it out of pressure or guilt. It's simply more fun for me than many other activities.
A healthy life outside of work and programming is important too. I don't spend every weekend at the keyboard, and I do try to sleep 8 hours a night. When I don't feel like going into the office, I don't. I'm my own boss and if I choose to the contribute to the world through a company I didn't happen to found, but do believe in, that's my prerogative. I don't know anyone at AirBnB, so I don't know, but I imagine the same may well have been true there too.