Correct me if I'm wrong, but can't mechanics do the same? I thought they were able to hold the vehicle and put a lein on it if the bill is unpaid?
Now, I believe towing is somewhat regulated so he might not be able to charge that much, but he could probably charge you $200+ in some areas.
But a mechanic is required to tell you what it will cost before the repairs start. When was the last time a doctor told you what the bill will be before they started?