And it does. Methods like solitary do classify as torture under international laws.
The not approved, but still happens, "code red"s that get employed by both staff and inmates, are very clearly torturous.
Slave labor, could also classify as torture. Especially when those meager wages are decimated by the basic costs of things. Such as the per-minute charges to read free ebooks that has been rolling out, or the ridiculous charges to connect by phone or email to anyone not living in the same hell that you are - something that has been shown to help the mental state of the incarcerated.