It's a terrible thing that happened but why is it the shops fault? You did sign the paperwork saying that it wasn't their responsibility, why would you go back after something that they clearly stated in their contract wasn't their fault? If you chose not to read the contract before signing(which a lot of us do)that is your fault. I'm glad you have insurance but if they were stolen, what is the dealership supposed to do, they had a clear contract of exactly what they were responsible for.
Keep in mind, I really do feel for you getting your mower stolen. I hate thieves! I have had things stolen myself. I would just pay the $250 and move on. It sucks for everyone involved, including the dealer.