First, let me say that what happened was horrible, and the person who did the damage should go to jail for a long long time. AirBnB should cover the cost of all damages as a gesture of goodwill.
It's really a shame that this kind of stuff can happen, but the reality (that I'd hope most people understand) is that not 100% of the population is good and decent (maybe it's 99%, maybe it's 98%, but it's certainly not 100%) Given that every mature adult is aware of this, the way the host acted was pretty irresponsible and was kind of inviting disaster.
She literally left a key for a total stranger to her home, without doing ANY due-diligence. Someone basically sent her an email, and she said "okey-dokey, here's the keys, have fun." What are the odds that if you do that 50 or 100 times that something like this doesn't happen? Also, how many people would be careless enough to do that?
Seriously, it's pretty tragic what happened and I really feel for the host, but it rubs me the wrong way that AirBnB is getting this massive amount of bad press when in fairness they just posted a listing and handled the transaction. Could they have more security measures in place? sure, but the way the host acted, I'm really not convinced anything would have made a difference here.
If you give keys to your place to a random stranger without so much as meeting them or getting basic information about them, then you're going to get burned.