I think you've answered your own question. Reducing lawsuits is not really the issue so much as the bad publicity that would result from a personal injury. The press love that sort of thing. There's no real way of putting a financial cost on that is there?
If there's turb reported by other aircraft in the vacinity then your crew are obviously wise to keep the sign on. Clear air turbulance is often expected under certain weather systems which by definition you won't see out your window and neither will the guys up front. However, they'll will be aware of it through having access to the latest weather forecast and radio contact with other aircraft in your area, either directly on the same frequency, or via air traffic control.
On other occasions your flight deck crew may have become preoccupied with the more important task of flying the aircraft and relegated the task of removing the seatbelt sign to further the down the list of priorities...... ie; forgotten it.
VFE.