A credit card number doesn't help much when you're dealing with anti-social elements (they'll just use a stolen one).
At the very least, it should be an option for property owners to purchase.
Even if you do have homeowners / renters insurance, I wouldn't be surprised if most policies have exclusions for operating your home as a rental.
If the value of the service is so low, and the risk so high, that it's essentially uninsurable, then perhaps that's a sign that it's a bad business model.
If you read the Airbnb TOS you'll see that they are well aware of this and are just hoping that naive customers don't read the whole thing.
I expect the original poster's losses will not be covered by home owner's insurance.
http://www.nysenate.gov/press-release/illegal-hotels-bill-pa...
Given how rare this type of thing is, AirBNB could have a great extra source of income by offering optional insurance for an extra fee (similar to car rentals).