Because there is a narrative to this story that AirBnb wants to portray, which may help them in their legal battles. It's the same reason why they call themselves part of the "sharing economy" - they're trying rub a grassroots, everyman shine all over the company, deservedly or otherwise.
Large, organized, professional operations run counter to this narrative. The story is supposed to be about helping the average Joe make ends meet, expect a lot of PR focus on working-class families, students, and the empowering effect of AirBnb income. Expect things that run counter to this narrative to be downplayed or eliminated.
Selfish? Sure. So is every other corporation on the planet.
I do think AirBnb should have been screening things better in the first place to prevent things from getting to this point. However, I'm sure that would have really slammed the breaks on their business model's growth.
Say that I sell pot on Etsy (which is illegal in NYC). Do I have a valid cause for complaining when Etsy shuts down my illegal business?
In the AirBnB case, the company deserves to be fined for knowing that 13% of their available listings are illegal and not trying to actively stop the offenses. The people profiting off the criminal behaviors deserve nothing.
To preempt what I imagine will be a response to this question, which is that these users are violating the TOS with Airbnb, I have two points: the first is that Airbnb itself is violating many "TOSs" - laws - so it doesn't give them very good moral standing here.
The second point is stronger (I think), which is that Airbnb's position here is logically inconsistent. They either believe strongly that you should be able to do what you want with the properties you own, or you don't. I simply don't see why it matters if N=1 or N=10 (put differently, N=1 is not a special case that deserves being considered differently).