There is a possibility of interference, but it's minimal. As to why the ped's aren't banned, in my experience the airlines feel that just telling people is enough. I've seen people go to the lav while the seatbelt sign is on and even while taxiing after landing. Rarely do the FA's even say anything about it.

On a Russian airliner I was flying on once a passenger went to the lav while taxiing and the pilot, for whatever reason, braked hard. I don't think there's any way that he knew the passenger was up, so I seriously doubt it was malicious. However, when the passenger complained about falling and cutting a nice little gash on his head, the FA and then the pilot basically told him he could find his own band-aid -- he should have known better than to be out of his seat with the seatbelt sign on. I and the rest of the passengers applauded.

Another incident with a laptop in a seatback pocket -- his seatmate gets up to go to the lav and the computer gets caught by his belt, flung across the aisle and breaks into pieces. Later at the gate I hear a supervisor tell him neither the airline nor the other passenger is responsible because the computer should not have been in the seatback pocket.

My opinion, you break the rules, the airline is relieved of all liability.

SR