Because it is against the law to tamper with fire equipment, including smoke detectors, and because smoke detectors are usually centrally located and unobstructed, employeres wishing to hide a camera in an employee locker room have often put it in the smoke detector. In fact there are companies who sell smoke detectors with built in cameras.

These cameras should be against the law for the same reason that tampering with smoke detectors is against the law. Such cameras encourage tampering with smoke detectors, obstructing smoke detectors, and the like, and result in increased fire hazards even where they are not used, simply because of suspected use.

Take a look at privacy issues of wearable cameras versus surveillance cameras.