Homeowners insurance covers the fire. They probably don't go after anyone unless they have specific reason to believe the loss was probably caused by a pattern of action that could result in a winnable lawsuit (good luck figuring that out when you have a melted lump of car to to work with). If someone does somehow go after the shop their insurance goes to bat for them and digs up all the reasons they're not liable and this was a reasonable repair that their customer performed.