The cast iron tea equipment is a Japanese development. In Japan, tea is most commonly a green variety, and brewed at a lower temperature than boiling. This can be as low as 104˚F (40˚C) for high grades of gyokuro, or as high as 167˚F (75˚C) for more common varieties of sencha.
The lower grade the tea, or the more oxidised/fermented the tea, the higher the temperature of the water recommended. In western tea culture, where black tea is the norm, the water used for tea preparation is usually boiling, 212˚F (100˚C), so if you are using cast iron equipment to prepare tea in the western fashion, you may be in for some burned fingers. This may also be the case for some Japanese ceramic tea services. I have one favourite Japanese stoneware tea cup that is equally likely to burn you if you put boiling water in it, unless you wait 5 minutes or more for it to cool.
In any case, the cost associated with cast iron tea services is such that they are more often used for more formal settings in which a higher grade of tea, and therefore, a lower water temperature, are more likely to be indicated.