If we all start leaving our vehicle keys in our unlocked vehicles and find that a significant rise in vehicle theft occurs, there's going to be those rare few that will place FULL blame on the thieves yet still make the observable connection between our actions and the calculable statistics.
Also, some of us will realize that the actions of the thieves who steal our cars and our actions leaving the keys in our unlocked vehicles are different actions, and crucially, that the moral blameworthiness of the thieves for their own actions is independent of any fault on our part. For example, when it comes to the thief blameworthiness, it doesn't matter if you left your keys in your unlocked vehicle out of negligence, or because someone else forced you to and the thief didn't know any of that, or because you're a police officer setting up a trap for a thief: the thief is exactly equally blameworthy for his action - namely, his attempt to steal your car, successful or not.
So, of course the thieves deserve full blame. Because it's not the case that there is a certain amount of blame to be distributed amongst the participants in a single action. Rather, there are different actions with different actors, and the blameworthiness of each of them for their own actions does not depend on the blameworthiness of the others for theirs.
Now, whether we would be morally blameworthy if we were to leave our keys like that is another matter. It seems clear that as long as we do not intend for anyone to attempt to steal our car, if we leave the car like that in a place where (like nearly all places) leaving the keys in an unlocked vehicle can be expected to raise significantly the chances of theft, we're being means-to-ends irrational, assuming no other relevant factors are at play (e.g., no one is forcing us). But do we have a moral obligation not to be means-to-ends irrational on the matter?
It depends. If - say - we depend on our car to do our job and make a living, and we have children in our care, then arguably yes, we would be morally blameworthy if we leave the keys like that. On the other hand, if we have no one in our care, and/or have billions of dollars and make money to buy several cars per hour, etc., it's more difficult to say. Maybe there is a general obligation to not make things easy for thieves? It's debatable. But in any event, regardless of whether we are morally blameworthy for our own action, the thieves are fully morally blameworthy for theirs.
But alas, in my experience, nearly everyone who considers matters like this fails to see any of the above.
