Well, nowhere does the contract between HTC, Tmobile, and the user say what the maximum temperature that this device can withstand does it? How are we to know how hot is too hot? A hari drier is not too hot for hair is it?
Legally, HTC would have to have forbid subjecting the phone to temperatures beyond certain values. You may disagree, but I assure you that this is how it works in USA courts. Why should he have known that the hair drier would damage the phone? In business, you can't just expect everyone to behave a certain way unless they agree to.
Just you wait, HTC will pull this same BS in their lawsuit with apple.