I know it says for use on glass only, but let's be honest. It's just a low power electric stove(basically) wrapped in non-electrically conductive material. It's a contact heating element. If you put it on a thermostat and have it heat to 90 degrees, well you'll be heating something at 90 degrees plain and simple.

If you run it uncontrolled it might melt through or burn the tub, which is why they say for use on glass only, otherwise their product is a fire hazard.