I can't say for snakes but for HUMANS the sensory nerves are different for heat (thermal) and for touch (mechanical). Something about reflexes going to the spinal cord (faster) versus impulses to the brain (slower). Obviously we can feel heat as pain, but maybe snakes can't. But when you think about it, when would a snake in the wild ever have to worry about burning it's belly on a 120 degree rock?
About the fires...I don't have any other info for you. :( It was very depressing for me to realize that there no clearly right (or safe) choice in the heating debate. However I doubt that these people were using thermostats to control the UTH. So using one gives me some sense of security. Also following package directions- not placing the tank on carpet or other flammable surface, not using it to heat non-glass material.