If the motel housekeeping or management is the one planting the camera, then the corporation will be held account. On the other hand, if a host plants the camera, AirBNB's language makes it clear that there is no accountability or responsibility on AirBNB's part. So the incentives to actually enforce that policy are entirely different.
Plus, the whole point of a hidden camera is that it's hidden. If anything, it would be far harder to find a hidden camera as the number of rooms to search increases.