Point of order ... not all 400Fs have 6 "cabin" seats.
Max occupancy on the 400F is 8, so some deem 4 cockpit seats & 4 "cabin" seats as being sufficient. (ie why have 10 seats when you can't put 10 backsides in them...)
Presumably the reason for the max occupancy is a licensing thing, rather than insurance, to insure the aircraft is definately used purely as a freighter?