If it gets hot, coolant becomes hotter too. Try opening your engine bay and checking the temperature of your car after driving for a long period of time or during the summer. There is a lot of engine heat and that heat is transferred to the coolant. If that coolant is transferred to the TB, then the TB is the same temperature as the motor. However, if coolant isn't running through it, the outside air it is sucking in will cool down the TB and therefore allow cooler air to pass through.