I'm puzzled as to why the property owners aren't suing them to recover all the money they lost. I mean, yeah, lawsuits cost money, too, but when you're talking a $20k fine, I would imagine there's at least a good chance of recovering more than you pay...and it didn't sound like any of the mentioned owners were attempting to do anything to the bad-faith tenants.
It sounds like you have never been a landlord. It's a five step process that takes at least two months just to evict someone in my landlord friendly state. I've heard it's worse in many other states. After that, it's possible to sue someone but that costs money and time and even after you do win it can take years to actually get your money and go through garnishment procedures once you track down their income. It's seldom worth the effort.
Yes most of the fault lies with AirBnb but being a distant landlord without a local property manager is crazy anyway. I sold my properties because the expense of getting a property manager or the hassle of managing it myself from across town wasn't worth it.