I’ve heard it’s because they’re enameled on the inside, but so is a Le Creuset so I’m confused. Maybe the enamel is of a different kind?
Any cast iron exterior will work, but there is an additional possible problem for induction users. My Staub cast iron has a base that is too narrow to activate induction. My electric hobs are slow. So, I use a Le Creuset enameled steel tea kettle, and pour the boiling water into my Staub cast iron. I used to use a Revere tea kettle that also worked perfectly with induction.
“Tetsubin (鉄瓶) are Japanese cast-iron kettles with a pouring spout, a lid, and a handle crossing over the top, used for boiling and pouring hot water for drinking purposes, such as for making tea.”
“Outside Japan, a frequently seen variant is a cast-iron teapot that outwardly resembles a tetsubin. This type of teapot is glazed with enamel on the inside to make it more practical for tea brewing, though it can’t be used to heat water because that would break the enamel coating. In the west, these teapots are commonly referred to as tetsubin, although the Japanese call them “tetsu kyūsu” (鉄急須), or iron teapot, to make a distinction from the kettle. Cast-iron teapots often come with a tea strainer that fits inside.”
As Amy has already mentioned, that “break the enamel coating” explanation is confusing–at best. My ECI Staub teapot is enameled CI on the outside and inside, and can be used to boil water–but a tea pot is really intended for the steeping phase in classical tea preparation. One pours boiling, or near boiling, water from a teakettle into a warm tea pot–with the tea leaves placed in an infuser–and one steeps the tea (keeps it warm) while the tea attains desired strength.
Unless it’s a really crappy pot, the interior enamel in a CI teapot shouldn’t crack while boiling water.
Like you said, a tea kettle is different from a teapot. Maybe the OP is confusing the two, I’m not sure. The OP asked about a specific tea pot, which I provided information for from a quick search online (which the OP probably could have found too). I don’t have a Staub tea pot or a Tetsubin tea pot. I use a basic $30 tea kettle and transfer the hot water to my mug or tea pot.