Shabbos 39 – שבת לט

Rashi explains the reason that cooking by exposure to the sun’s rays is permitted since it is not ‘derech bishul’ (ordinary method of cooking).  The Gemara tells us that according to R’ Yosi cooking with Tiberian hot-springs is considered an issur min hatorah since they are heated by the fire of Gehinom.  Why isn’t that too considered not ‘derech bishul’?