I agree - maybe I'm just drinking the YC gatorade here, but to me it shows scrappiness and dedication and does not seem especially immoral. Who did it hurt? For the people who were annoyed by the spam, it took 30 seconds out of their life to read and delete the email. For the ones who went ahead and posted their rental on airbnb, it probably led to more business plus the added benefit of getting to use airbnb's elegant interface instead of trying to keep track of dozens of craigslist emails. The only money that airbnb made from this lie was by finding people additional renters for their homes, which seems like an everybody wins scenario.

If it was outright illegal then they'll have to face the repercussions of that, but I doubt they'll lose much sleep feeling bad about what they've done.