Steve Lehto is a Lawyer in Michigan who specializes in Automotive law. He regularly writes for Jalopnik and also has a regular podcast that covers issues such as this. If I remeber correctly he claims that the waivers that companies ask you to sign when they are in possession of your vehicle are total B.S. and that the owner of the business is completely liable for any damage that occurs during that time. I dont remember which podcast covers this but here is a link. They are all quite interesting.
Podcast Archive - Law Office of Steve Lehto