Unfortunately, it's not just about how hot the mat gets, it's about how long the snake is in contact with it.
Snakes' nervous systems don't seem to register heat in the same way that a mammal's would. For example, spotlights in vivs. If a human touches the bulb of a spotlight, we'll immediately register that it's burning hot and move our hand away to minimise our injury. However, there are numerous examples where Corns (and many other reptiles) have gone and sat on unguarded, searing hot, bulbs in vivs for some time and been horribly burned as a result.
It's quite possible for a Corn to go and sit on (or under) an unregulated heat mat for a long time, without sensing that it's in danger of injury.
Basically, we can't trust the snake's instinct to preserve itself when it comes to artificial heat sources. It hasn't evolved the facilities to deal with them. We have to take control of that aspect of its welfare by always using a thermostat.