Do a google img search for belly burned snakes. It won't be pretty. Then imagine Marshal suffering that way. Why argue about it? It's for the safety of your pet! These people don't have to take time to try to advise you. They have practical experience and are trying to save you and your darling snake very avoidable pain.
When multiple breeders/keepers some of whom have been caring for ball pythons longer than you've existed take the time to explain to you that a thermostat will help you care for your snake better than the great care you're already giving it, it might be a good idea to follow their advice.
