In fact, almost all BB cups come with a ring of threadlocker on the threads from new. This stuff is anaerobic (sets in the absence of air, i.e. when the cups are installed) and, like DT spoke-freeze, will set up even if you have used a lubricant on the threads. If the cups are being reused, and the threadlocker is gone, a reapplication of a small amount in addition to grease on the BB shell and proper torque will ensure it torques properly, can be removed when desired, but stays in place. It's mostly correct torque that holds them in, but the threadlocker helps a bit.